chore: harden GHA workflows

[nscuro]

• adds cooldown period of 7 days for Dependabot
• bumps all 3rd party actions to their latest version and pins them by digest
• disables persist-credentials option for actions/checkout
• removes unnecessary permissions:
  • contents: read is implied for public repos
  • statuses: write is not required by amannn/action-semantic-pull-request
• migrates semantic-pr.yml away from pull_request_target, pull_request does the intended job just fine
• enforces timeouts across all jobs
• adds zizmor job to detect regressions

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