ci: check only PR's in actions/stale

[DmitriyLewen]

Description

We no longer need stale issues action, because maintainers are opening up issues.
Use actions/stale for PR's only (actions/stale supports -1 value for days-before-stale - days-before-stale).

Test run - https://github.com/DmitriyLewen/trivy-issue-style-test/actions/runs/6415444244/job/17417366537

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).