doth: enable max-age checking in system test on OpenBSD

OpenBSD's curl has HTTP/2 support but because of differences in grep implementation, a check in doth test disables max-age checking tests which need HTTP/2 aware curl and are skipped with The available version of CURL does not have HTTP/2 support message.

curl -V:

curl 7.79.0 (x86_64-unknown-openbsd7.0) libcurl/7.79.0 LibreSSL/3.4.1 zlib/1.2.11 nghttp2/1.44.0
Release-Date: 2021-09-15
Protocols: dict file ftp ftps gopher gophers http https imap imaps mqtt pop3 pop3s rtsp smb smbs smtp smtps telnet tftp 
Features: alt-svc AsynchDNS HSTS HTTP2 HTTPS-proxy IPv6 Largefile libz NTLM NTLM_WB SSL UnixSockets

But the current check won't detect it:

$ /usr/local/bin/curl --version | grep '^Features:.* HTTP2\( \|$\)'
$ 

This should do (also tested on Fedora 35 & FreeBSD 12.3):

$ /usr/local/bin/curl --version | grep -E '^Features:.* HTTP2( |$)'
Features: alt-svc AsynchDNS HSTS HTTP2 HTTPS-proxy IPv6 Largefile libz NTLM NTLM_WB SSL UnixSockets

The check originated here: !5585 (comment 250222).