repoview: flag `server.view` as experimental

Ideally, the non-experimental version of `experimental.extra-filter-revs` will
cover the use case for `server.view=immutable` well enough than having to have
this dedicated configuration. Since `server.view` is not part of any release, I
would prefer to have it marked as experimental to avoid having it to support it
for ever.

mercurial/help/config.txt

@@ -2050,7 +2050,8 b' Controls generic server settings.'
     Repository filter used when exchanging revisions with the peer.
 
     The default view (``served``) excludes secret and hidden changesets.
-    Another useful value is ``immutable`` (no draft, secret or hidden changesets).
+    Another useful value is ``immutable`` (no draft, secret or hidden
+    changesets). (EXPERIMENTAL)
 
 ``smtp``
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