Add Missing Input Reading for only-issue-types

[Bibo-Joshi]

Description:
Follow up for #1255. Turns out the added functionality was not working because the input was not read. This closed a number of issues for me which I had to repoen 😅
I manually tested the changes proposed here (see python-telegram-bot/python-telegram-bot#5005), so that should hopefully cover it.
Note that I did not add any new unit tests because I also did not find any tests that check this for the other inputs. Maybe a separate issue/PR could track that there is a centralized tests that ensures that all input is respected …
Looking forward to your feedback :)

Related issue:
PR: #1255
Re-closes #1187

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[Copilot]

Pull Request Overview

Fixes missing input reading for the only-issue-types parameter that was introduced in PR #1255. The functionality was not working because while the parameter was defined, the input value was never actually read from the GitHub Actions input.

• Adds the missing core.getInput('only-issue-types') call to read the input value
• Ensures the only-issue-types feature works as intended

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[Bibo-Joshi]

kindly pinging @luketomlinson as you've been involved in merging #1255 :)