Add reusable workflow for updating action version in release PRs

[marians]

Summary

• Adds update-action-version.yaml reusable workflow that finds an open release PR, extracts the version, and updates the specified composite action's action.yml file
• Replaces per-repo JS scripts (find_release_pr/index.js) with simple shell commands using gh CLI and jq
• Accepts action_yml_path (required) and yq_expression (optional, defaults to .runs.steps[0].with.version) as inputs

Resolves giantswarm/roadmap#4052

Test plan

• Merge this PR first, then merge the caller PRs in helm-values-gen and schemalint
• Trigger a release in helm-values-gen and verify the version in actions/ensure-generated/action.yml gets updated
• Trigger a release in schemalint and verify the version in actions/verify-helm-schema/action.yml gets updated

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