Streamline Release Process by Removing Release Branch Requirement

[shaynafinocchiaro]

Is this a new chart?

No

What this PR does / why we need it:

This PR updates the charts release action to be manually triggered so we can merge to the main branch without triggering the release workflow. We will no longer need to create a "release PR" in order to kick off the release workflow. We no longer need to merge to a release branch either.

1. Updates the target branch for dependabot to main branch.
2. Removes post-release-action.yml and rebase-release.yml because they are no longer needed now they we can merge to main

Tested change in forked repository: https://github.com/shaynafinocchiaro/helm-charts-test/actions/runs/13207643634

Which issue(s) is this PR associated with:

• [FEATURE]: Automate the release process for CSM csm#1490

Special notes for your reviewer:

Checklist:

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• Chart Version bumped
• Variables are documented in the chart README.md
• Title of the PR starts with the chart name (e.g. [charts_dir/mychartname]) if applicable