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GH-39204: [Format][FlightRPC][Docs] Stabilize Flight SQL (#41657)

Update documentation, protobufs, and class documentation to remove experimental tags from Flight and Flight SQL documentation.

### Rationale for this change
Flight SQL has been used by multiple databases now and has been voted as stable per the
mailing list discussion: [https://lists.apache.org/thread/qoshg8mln3t2ovr90o1yklz4yrpv503h](url)

### What changes are included in this PR?
Update protobuf, class comments, and user documentation to remove references to Flight and Flight SQL being experimental. This change excludes the UCX transport and the session option messages

### Are these changes tested?
No, documentation only.

### Are there any user-facing changes?
User documentation.
* GitHub Issue: #39204

Authored-by: James Duong <james.duong@improving.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>