DNS-over-TLS should use "dot" ALPN

Although [RFC7858] does not seem to mention any specific ALPNs for DoT, the RFC9103 for XoT mentions that dot MUST be used (see section 7.1).

During connection establishment the Application-Layer Protocol Negotiation (ALPN) token "dot" [DoT-ALPN] MUST be selected in the TLS handshake.

Thus, we should just use dot as ALPN for all DoT connections as it is registered as ALPN for DoT.

This issue is a part of #2775 (closed).