ci: Move db820c and db410c's gles3 tests to manual, like radv did.

ci: Move db820c and db410c's gles3 tests to manual, like radv did.

This should make these tests available for clicking on the web ui in
personal branches, while hiding them from marge and the post-merge CI
pipelines.  We had already disabled db410c's gles3, but it wasn't
available in the ui and you had to hack .gitalb-ci.yml.  db820c is now
being disabled by default, due to instaboots mentioned in
https://gitlab.freedesktop.org/mesa/mesa/issues/2649

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4247>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4247>