Update GH Actions to align to new guidelines

[QcFe]

Description

This PR changes the dramatic set-output::stuff to the new >> $GITHUB_OUTPUT, in order to remove deprecation warnings and align to new GH Actions format. This also updates retrieve-build-matrix custom action to latest version of @actions/core.

[kingmakerbot]

Hi @QcFe. Thanks for your PR.

I am @kingmakerbot. You can interact with me issuing a slash command in the first line of a comment. Currently, I understand the following commands:

• /rebase: Rebase this PR onto the master branch
• /merge: Merge this PR into the master branch
• /hold: Adds hold label to prevent merging with /merge
• /unhold: Removes the hold label to allow merging with /merge
• /deploy-staging: Deploy a staging environment to test this PR (the build-all flag enables user environments building)
• /undeploy-staging: Manually undeploy the staging environment

Make sure this PR appears in the CrownLabs changelog, adding one of the following labels:

• kind/breaking: :boom: Breaking Change
• kind/feature: :rocket: New Feature
• kind/bug: :bug: Bug Fix
• kind/cleanup: :broom: Code Refactoring
• kind/docs: :memo: Documentation

[frisso]

Many many thanks, @QcFe !

[giorio94]

@QcFe Since you are playing with GH Actions, it might be worth to bump a few actions, to remove also the warnings about the deprecated node version (and probably also the remaining ones about save-state)...

[QcFe]

/deploy-staging

[kingmakerbot]

Your staging environment has been correctly deployed/updated! Available here: Frontend, Qlkube Operators: add the crownlabs.polito.it/operator-selector=staging-836 label to your tenant.

[QcFe]

/merge

[kingmakerbot]

Your staging environment has been correctly teared-down!