An attempt to build with pg15 and nighly timescale

[sumerman]

Description

DO NOT MERGE

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

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

[sumerman]

Closing this one in favor of https://github.com/timescale/promscale_extension/pull/604

Thou shall not pgXX in thine branch names useth lest thou connector's e2e wrath seeketh