CXX-3323 drop support for MongoDB Server 4.0 or older

[eramongodb]

Resolves CXX-3323. Followup to #1429 and #1434.

#1434 already added the corresponding CHANGELOG entry and removed related EVG tasks.

Scans the codebase for all instances of MongoDB Server version checks against a value that corresponds to 4.0 or older and removes them. Most of these are in test code, but there are a few bits of library code as well, such as in mongocxx::v_noabi::collection::_drop() and an obsolete doc comment for mongocxx::v_noabi::result::bulk_write::modified_count().

[kevinAlbs]

LGTM with some suggested additional removals:

• Calls test_util::newer_than (which may be better named to test_util::equal_or_newer_than) for "4.2" and older.
• This check for commitQuorum support.

[eramongodb]

which may be better named to test_util::equal_or_newer_than

Opted for test_util::server_version_is_at_least to match its description:

Returns 'true' if the server version for the default client is at least 'version', returns 'false' otherwise.