Make backporting action use PRs to support protected branches [DOC-268]

[JackPGreen]

In DOC-255, branch protection was added - preventing direct pushes.

Instead, the backport workflow should create PRs to backport the changes. This gives the possibility for pre-merge checks to be run (i.e. preventing introducing dead links etc).

Leverages existing backport tool, but exposed some issues which need to be addressed before this can be merged:

• https://github.com/hazelcast/backport/pull/14
• https://github.com/hazelcast/backport/pull/15
• https://github.com/hazelcast/backport/pull/16

Tested in a test repo, where a dummy PR was successfully backported. However, due to the complexity of fork-triggering, it wil need to be re-tested once merged.

Fixes: DOC-268

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