ci(gitlab-ci): deprecate `3000.9` (and remove Jinja2 downgrade workaround where possible)

• https://saltproject.io/salt-platform-support/
  • Phase 3 support ends: Aug 31, 2021
• With this commit, we're no longer building Python 2 pre-salted images

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This MR also removes the Jinja2 downgrade workaround where possible.

Fix has been released:

• https://github.com/saltstack/salt/pull/60811

Maintain Jinja2 downgrades where necessary, as outlined here:

• https://github.com/saltstack/salt/pull/60811#issuecomment-911772632