Replace explicit token input with implicit GITHUB_TOKEN [DI-690]

[JackPGreen]

The action currently requires a token input, which means a typical implementation uses a long-lived PAT:
https://github.com/hazelcast/hz-docs/blob/b01fcb2b18c8cd4eb87dc901ca6972ad9832f1a9/.github/workflows/backport-workflow.yml#L37-L39, and requires pull_request_target execution for that secret to be accessible.

Instead, we should use the default GITHUB_TOKEN to avoid this.

Example execution and resultant PR - note now it's not using devOpsHazelcast's token, PRs will instead by github-actions[bot].

Fixes: DI-690

Post-merge actions:

• update every reference (...including maintenance branches...) as per https://github.com/hazelcast/hazelcast-mono/pull/5723

[nishaatr]

would have preferred one PR tbh
little confusing

[JackPGreen]

would have preferred one PR tbh little confusing

What do you mean?

[JackPGreen]

would have preferred one PR tbh little confusing

What do you mean?

Now #33 is merged and this branch is updated it should be clearer.