Note discovery-file's deprecation in release notes

[DaveCTurner]

This change adds a note about the deprecation of the discovery-file plugin to
the release notes for 6.5. Relates #33241.

[elasticmachine]

Pinging @elastic/es-distributed

[DaveCTurner]

@lcawl my main concern here is that this PR introduces the 6.5 release notes and removes the commented-out text at the top of the 6.4 release notes. I'm not sure what the process is for that but this seemed like the best thing to do.

[lcawl]

@DaveCTurner I ran a build on this content and it looks good to me. There are some items missing from the 6.4 release notes, but I'll forward-fit those in a separate PR.
Please note: If you haven't already done so, can you add a more verbose description of the breaking changes in this section too?: https://www.elastic.co/guide/en/elasticsearch/reference/6.x/breaking-changes-6.5.html

[DaveCTurner]

The failure of the JDK11 build is unrelated to this change; I opened #33579.

[ywelsch]

LGTM

[DaveCTurner]

@lcawl I think there are no breaking changes here - the associated breaking change is #33257 which includes its own additions to the release notes.

[lcawl]

LGTM