fix: upgrade stac-fastapi to v6.1.5

[ividito]

Issue

#497

What?

• Update PG env var names
• Add flag to create writer connection pool for transactions
• Add a bunch of new tests for transactions, which also makes local OIDC work better

There were inconsistencies between how unit tests were running and how the actual service was behaving with the transactions changes. I had a really hard time getting meaningful errors out of the unit tests, so I converted them into integration tests that run against the docker compose services and make use of the existing auth integrations we have there. This was way more promising and has me feeling confident about the changes, but I'm still not sure what's wrong with the unit tests.

Testing?

• Tests pass Unit tests fail 🤔, manual + integration tests pass
• Manually tested transactions, since the new default behaviour is to enable transactions with a new stac-fastapi environment variable. veda-backend already has a variable to enable the extension, which duplicates this functionality. We can continue to use this approach without breaking existing instances, or we can migrate to the alternative stac-fastapi variable.

[ividito]

Unit tests fixed (thanks @smohiudd ), integration test changes moved over to #563