ci: Make "fast-ci" the default validate configuration

We are trying out a lighter weight validation pipeline where by default
we just test on 5 platforms:

* x86_64-deb10-slow-validate
* windows
* x86_64-fedora33-release
* aarch64-darwin
* aarch64-linux-deb10

In order to enable the "full" validation pipeline you can apply the
`full-ci` label which will enable all the validation pipelines.

All the validation jobs are still run on a marge batch.

The goal is to reduce the overall CI capacity so that pipelines start
faster for MRs and marge bot batches are faster.

Fixes #23694