github: Adjust settings of the stale workflow

[radeksimko]

This PR proposes a few changes to the stale GHA workflow, which was effectively unused due to one of the settings set more conservatively.

Specifically, we set days-before-stale: '-1' which means that the bot would never attempt to mark issues as stale by itself. That first step was left to us, but I think nobody even realised this is the case and so nobody ever assigned stale label to an issue/PR.

With some older issues being accumulated in the repo, I wanted to update the settings, such that we can more proactively tackle the backlog and communicate expectations to the contributors/reporters.

• all automation remains scoped to just issues labelled waiting-response
• bot will automatically label an issue/PR as stale after 30 days of no response and comment with an updated message, which also states the inactivity period
• bot will automatically close an issue/PR if it was stale for over 30 days and received no response, and comments with a message explaining why

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.