Fix Cloud Extensions Regression (#11907)

## Problem
The regression test on extensions relied on the admin API to set the
default endpoint settings, which is not stable and requires admin
privileges. Specifically:
- The workflow was using `default_endpoint_settings` to configure
necessary PostgreSQL settings like `DateStyle`, `TimeZone`, and
`neon.allow_unstable_extensions`
- This approach was failing because the API endpoint for setting
`default_endpoint_settings` was changed (referenced in a comment as
issue #27108)
- The admin API requires special privileges.
## Summary of changes
We get rid of the admin API dependency and use ALTER DATABASE statements
instead:
**Removed the default_endpoint_settings mechanism:**
- Removed the default_endpoint_settings input parameter from the
neon-project-create action
- Removed the API call that was attempting to set these settings at the
project level
- Completely removed the default_endpoint_settings configuration from
the cloud-extensions workflow
**Added database-level settings:**
- Created a new `alter_db.sh` script that applies the same settings
directly to each test database
- Modified all extension test scripts to call this script after database
creation

docker-compose/ext-src/alter_db.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+# We need these settings to get the expected output results.
+# We cannot use the environment variables e.g. PGTZ due to
+# https://github.com/neondatabase/neon/issues/1287
+export DATABASE=${1:-contrib_regression}
+psql -c "ALTER DATABASE ${DATABASE} SET neon.allow_unstable_extensions='on'" \
+     -c "ALTER DATABASE ${DATABASE} SET DateStyle='Postgres,MDY'" \
+     -c "ALTER DATABASE ${DATABASE} SET TimeZone='America/Los_Angeles'" \