PostgreSQL 15 Upgrade prep

[sumerman]

Description

Current status:

• The extension builds and cargo pgx tests pass.
• Dump-restore passes
• Upgrade tests for pg15 are disabled because there are no previous versions of promscale_extension built for pg15.
• ~We agreed not to test/publish alpine image for pg15~ I figured out a quick & dirty workaround.
• The PG15 package for Bionic is not tested because there's no corresponding TSDB package.
• Making the switch to timscaledev/timescaledb-ha because we need to release our side first before timescale/timescaledb-ha for pg15 can be released.

Merge requirements

Please take into account the following non-code changes that you may need to make with your PR:

• [x] CHANGELOG entry for user-facing changes
• [ ] Updated the relevant documentation