Reapply "build(deps): bump diesel to 2.3, async-bb8-diesel to 0.3.0 and bb8 to 0.9" (hold until hyperswitch moves)

[maverox]

Re-lands the diesel 2.3 bump from #91, which #95 reverts.

Not ready to merge. This is held open deliberately so the work is not lost
and does not need rewriting. It should land only once hyperswitch has moved off
diesel 2.2.10 — see #95 for why: deja-diesel is compiled into hosts that pin
their own diesel, and the trait surface it wraps is exempt from diesel's semver
promise, so this pin has to move with the host rather than ahead of it.

What has to be true before merging

• hyperswitch resolves diesel 2.3.x, async-bb8-diesel 0.3.0 and bb8 0.9
  (today on main it resolves 2.2.10 / 0.2.1 / 0.8.6)
• set_prepared_statement_cache_size — re-added here — is present on the
  Connection trait of whichever diesel version the host lands on

Mechanics. #91 is merged, and GitHub does not allow reopening a merged pull
request, so this is a new one carrying the same change. Sanchith's original
branch bump-diesel-2.3 was deleted on merge and has been restored at
d1b9a9c, so the original commits remain reachable.

This is stacked on revert/diesel-2.3-bump (#95) so its diff shows only the
bump; when #95 merges, GitHub retargets this to main automatically.

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