ci: improve fetch Core manifest workflow

[anmazzotti]

What this PR does / why we need it:

This PR introduces various improvements around the Core CAPI Provider manifest update workflow and related tooling.
In particular:

1. go.mod updated to ignore other unrelated projects
2. remove redundant crust-gather binary installation and default to krew
3. always install the clusterctl-operator when configuring kubectl and krew
4. cleanup fetch-core-capi.sh script
5. add make update-core-capi-manifest for easy usage
6. cleanup github action workflow and remove cron trigger, this is on-demand job only
7. dump unzipped manifest as artifacts on update jobs

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2009

Special notes for your reviewer:

Checklist:

• squashed commits into logical changes
• includes documentation
• adds unit tests
• adds or updates e2e tests

[salasberryfin]

Thanks @anmazzotti.

It is way cleaner now.