remove triggering mechanism for pull_request and add periodic tests

[thunderboltsid]

• pull_request_target is now the main pull request triggering mechanism for secure workflow executions
• remove conformance and cluster-upgrade-conformance from presubmit e2es
• add conformance and cluster-upgrade-conformance as periodic e2es

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 31.74%. Comparing base (95c177e) to head (9c5d976).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #454 +/- ## ======================================= Coverage 31.74% 31.74% ======================================= Files 14 14 Lines 1367 1367 ======================================= Hits 434 434 Misses 933 933 ```

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