Add deprecation of NFS Support to 15.6 release post

Why is this change being made?

The NFS Deprecation statement was accidentally omitted from the 15.6 release post. As such, this MR is being raised to add the statement as an extra content block to ensure our customers understand that the removal of NFS support has occurred as of November 22, 2022 (to coincide with 15.6).

The issue outlining this removal can be see here - NFS Support Deprecation on November 22, 2022 (gitlab-org/gitlab#351243 - closed)

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