VerifyPagesDomain: suffix with . indicate absolute fqdn

What does this MR do and why?

Append . to domain_name (if not present) in check function of VerifyPagesDomainService before performing DNS queries with Resolver. This ensures that the FQDN is treated as an absolute query, bypassing search list and ndots specification.

IETF RFC 1034, section 3.1 clarifies:

Since a complete domain name ends with the root label, this leads to
a printed form which ends in a dot.  We use this property to distinguish
between:

   - a character string which represents a complete domain name
     (often called "absolute").  For example, "poneria.ISI.EDU."

Relates to gitlab-org/charts/gitlab#3008

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