PR: https://github.com/mongodb/mongoid/pull/5762

Currently, the minimum MongoDB server version which Mongoid supports is 3.6. This PR removes references to 3.4 and prior.

Note that:

• This PR only affects documentation and tests; it does not alter the lib code itself.
• This PR does not alter/drop support for any MongoDB server version that has not already been dropped (in other words, it keeps the status-quo of support.)

Includes:

• Make documentation of section "Embedded Matching vs Server Behavior" more terse in light of the changes.
• Update list of supported auth_mechs (out-of-sync with driver, does not reflect latest server support)
• Remove various references to server versions 3.4 and prior in docs and test definitions.