Remove CentOS 6 from GitLab CI

CentOS 6 reaches EOL on November 30, 2020 and will not be officially
supported by the CentOS project.
77 jobs for !4392 with 2274-drop-centos-6-support-after-november-30-2020 in 56 seconds (queued for 7 seconds)
detached
Status Job ID Name Coverage
  Precheck
passed #1327437
amd64 linux
autoreconf

00:00:17

passed #1327439
amd64 linux
clang-format

00:00:19

canceled #1327440
amd64 linux
coccinelle

00:00:54

passed #1327441
amd64 linux
danger

00:00:16

passed #1327438
amd64 linux
misc

00:00:14

 
  Build
canceled #1327454
amd64 linux
clang:asan

00:00:31

canceled #1327458
amd64 linux
clang:buster:amd64

00:00:35

canceled #1327461
amd64 libvirt
clang:freebsd11:amd64

00:00:36

canceled #1327462
amd64 libvirt
clang:freebsd12:amd64

00:00:29

canceled #1327463
amd64 libvirt
clang:openbsd:amd64

00:00:27

canceled #1327456
amd64 linux
clang:tsan

00:00:31

canceled #1327442
amd64 linux
gcc:alpine3.12:amd64

00:00:36

canceled #1327453
amd64 linux
gcc:asan

00:00:32

canceled #1327446
amd64 linux
gcc:buster:amd64

00:00:35

canceled #1327443
amd64 linux
gcc:centos7:amd64

00:00:36

canceled #1327444
amd64 linux
gcc:centos8:amd64

00:00:35

canceled #1327452
amd64 linux
gcc:focal:amd64

00:00:32

canceled #1327457
amd64 linux
gcc:mutexatomics

00:00:31

canceled #1327448
amd64 linux
gcc:out-of-tree

00:00:34

canceled #1327447
amd64 linux
gcc:sid:amd64

00:00:35

canceled #1327449
i386 linux
gcc:sid:i386

00:00:34

canceled #1327459
amd64 linux
gcc:softhsm2.4

00:00:35

canceled #1327460
amd64 linux
gcc:softhsm2.6

00:00:34

canceled #1327445
amd64 linux
gcc:stretch:amd64

00:00:35

canceled #1327455
amd64 linux
gcc:tsan

00:00:31

canceled #1327450
amd64 linux
gcc:tumbleweed:amd64

00:00:33

canceled #1327451
amd64 linux
gcc:xenial:amd64

00:00:33

canceled #1327465
amd64 windows
msvc:windows:amd64

00:00:52

canceled #1327464
amd64 linux
nolibtool:sid:amd64

00:00:30

 
  Unit
canceled #1327477
amd64 linux
unit:clang:asan

canceled #1327481
amd64 linux
unit:clang:buster:amd64

canceled #1327484
amd64 libvirt
unit:clang:freebsd11:amd64

canceled #1327485
amd64 libvirt
unit:clang:freebsd12:amd64

canceled #1327479
amd64 linux
unit:clang:tsan

canceled #1327466
amd64 linux
unit:gcc:alpine3.12:amd64

canceled #1327476
amd64 linux
unit:gcc:asan

canceled #1327470
amd64 linux
unit:gcc:buster:amd64

canceled #1327467
amd64 linux
unit:gcc:centos7:amd64

canceled #1327468
amd64 linux
unit:gcc:centos8:amd64

canceled #1327475
amd64 linux
unit:gcc:focal:amd64

canceled #1327480
amd64 linux
unit:gcc:mutexatomics

canceled #1327471
amd64 linux
unit:gcc:sid:amd64

canceled #1327472
i386 linux
unit:gcc:sid:i386

canceled #1327482
amd64 linux
unit:gcc:softhsm2.4

canceled #1327483
amd64 linux
unit:gcc:softhsm2.6

canceled #1327469
amd64 linux
unit:gcc:stretch:amd64

canceled #1327478
amd64 linux
unit:gcc:tsan

canceled #1327473
amd64 linux
unit:gcc:tumbleweed:amd64

canceled #1327474
amd64 linux
unit:gcc:xenial:amd64

canceled #1327486
amd64 linux
unit:nolibtool:sid:amd64

 
  System
canceled #1327498
amd64 linux
system:clang:asan

canceled #1327502
amd64 linux
system:clang:buster:amd64

canceled #1327505
amd64 libvirt
system:clang:freebsd11:amd64

canceled #1327506
amd64 libvirt
system:clang:freebsd12:amd64

canceled #1327500
amd64 linux allowed to fail
system:clang:tsan

canceled #1327487
amd64 linux
system:gcc:alpine3.12:amd64

canceled #1327497
amd64 linux
system:gcc:asan

canceled #1327491
amd64 linux
system:gcc:buster:amd64

canceled #1327488
amd64 linux
system:gcc:centos7:amd64

canceled #1327489
amd64 linux
system:gcc:centos8:amd64

canceled #1327496
amd64 linux
system:gcc:focal:amd64

canceled #1327501
amd64 linux
system:gcc:mutexatomics

canceled #1327492
amd64 linux
system:gcc:sid:amd64

canceled #1327493
i386 linux
system:gcc:sid:i386

canceled #1327503
amd64 linux
system:gcc:softhsm2.4

canceled #1327504
amd64 linux
system:gcc:softhsm2.6

canceled #1327490
amd64 linux
system:gcc:stretch:amd64

canceled #1327499
amd64 linux allowed to fail
system:gcc:tsan

canceled #1327494
amd64 linux
system:gcc:tumbleweed:amd64

canceled #1327495
amd64 linux
system:gcc:xenial:amd64

canceled #1327507
amd64 linux
system:nolibtool:sid:amd64

 
  Docs
canceled #1327508
amd64 linux
docs

00:00:29

 
  Postcheck
passed #1327512
amd64 linux
cppcheck

00:00:23

passed #1327509
amd64 linux
flake8

00:00:21

canceled #1327513
amd64 linux
gcov

passed #1327510
amd64 linux
pylint

00:00:23

canceled #1327511
amd64 linux
scan-build

00:00:26