Adjust concurrency in gha workflow

[beni0888]

Description of the change

With these changes we achieve the following:

• Do not trigger the Kubeapps Main Pipeline workflow for every push, only for pushes to the master branch.
• Use a different concurrency group for the Release Pipeline, so a later workflow triggered from a push to the main branch won't cancel the release pipeline.
• Do not trigger the Kubeapps Main Pipeline workflow on tags creation. We trigger the release pipeline for this event and don't want a second pipeline running on the same version of the code, that could interfere with the former (eg. canceling the release pipeline because they both run in the same concurrency group).

Benefits

• Less number of pipelines triggered, and the triggered ones are more meaningful.
• A more resilient release flow.

Possible drawbacks

• The main pipeline isn't triggered until a PR is triggered so the feedback loop could be larger.

Applicable issues

• related to Migrate CI to Github actions #4436

Additional information

N/A

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[antgamdia]

Thanks!