Add support for multiple regular expressions in `prune-tags-regexes`

[vlaurin]

For example:

steps:
  - name: Prune
    uses: vlaurin/action-ghcr-prune@main
    with:
      token: ${{ secrets.YOUR_TOKEN }}
      organization: your-org
      container: your-container
      prune-tags-regexes: |
        ^pr-
        ^test-

Given Github actions currently do not natively support YAML lists as input value, multi-value inputs can be achieved using multi-lines, like it's done in Github's own actions/cache.
This is supported by the action toolkit getMultilineInput().