Rename some GUCs

As noted by @jonels-msft:
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Also, some thoughts about naming. Is it possible to rename some of these GUCs? Is there a backward compatible way to do that, like make an alias for a GUC, deprecate the old name, and finally remove the old name in the next major version?
If so, what do you think of these changes:
current proposed reason
max_adaptive_executor_pool_size max_session_conns_per_worker Communicate that it’s per-session, per-worker.
max_shared_pool_size max_conns_per_worker Once again, per-worker, but across sessions.
(Also, the proposed names have parity with max_cached_conns_per_worker. Although for consistency maybe that should be max_cached_session_conns_per_worker…)

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