[20.0][CI] Configure pgvector/pgvector:pg16 for PostgreSQL service in CI

[BhaveshHeliconia]

This PR switches the PostgreSQL CI image to pgvector/pgvector:pg16 in .copier-answers.yml and .github/workflows/test.yml.

The field_vector module requires the PostgreSQL pgvector extension, which isn't available in the default postgres:16 image. This causes the CI tests to fail when the module tries to install the extension.

Using the pgvector image fixes this issue.

Related PR: #3791

[BhaveshHeliconia]

Hey @OCA/tools-maintainers would be great if someone could have a look at this.

[thomaspaulb]

@sbidoul Do you know why in this PR we have:

2026-10-16 07:36:19,081 287 WARNING ? odoo.tools.config: option addons_path, invalid addons directory '/__w/server-tools/server-tools', skipped
2026-10-16 07:36:19,081 287 WARNING ? odoo.tools.config: option addons_path, invalid addons directory '/__w/server-tools/server-tools', skipped

But in another recent 20.0 CI job we don't?

I can debug more, but maybe you have seen it before.

[sbidoul]

I think it's a regular Odoo warning when a directory in addons path does not contain any addon.