feat(NODE-6278): deprecate 3.6 servers

[baileympearson]

Description

What is changing?

This PR adds a readme change primarily so that we have a PR for which to add release highlights. We must communicate to users at least a minor version ahead of dropping support for a server version.

Is there new documentation needed for these changes?

What is the motivation for this change?

Release Highlight

3.6 Server Support Deprecated

Support for 3.6 servers is deprecated and will be removed in a future version.

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket