Postgis driver cleanup and bug fix

[SpacemanPaul]

Reason for this pull request

• Postgis driver is still reporting itself as "experimental" and "unstable"
• Count methods in postgis driver did not support geospatial search.

Proposed changes

• Remove warning from postgis driver.
• Add geospatial search to count methods in postgis driver
• cherrypick COG fixes from 1.8 - will probably replace with odc-geo implementation before final 1.9.0 release.

NOTE: MacOS/Windows conda smoke test failures appear to be due to infrastructure issues rather than the PR.

• [x] Tests added / passed
• [x] Fully documented, including docs/about/whats_new.rst for all changes