[ci] lock inactive issues and PRs at different time

[StrikerRUS]

Our workflow for locking inactive issues and PRs often fails (https://github.com/microsoft/LightGBM/actions/workflows/lock.yml) with the following error:

Request quota exhausted for request GET /search/issues
Retrying after 60 seconds
Request quota exhausted for request GET /search/issues
Retrying after 60 seconds
Request quota exhausted for request GET /search/issues
Retrying after 60 seconds
Request quota exhausted for request GET /search/issues
Error: You have exceeded a secondary rate limit. Please wait a few minutes before you try again. If you reach out to GitHub Support for help, please include the request ID 9C00:2E562D:2A4052F:538F1F4:676[21](https://github.com/microsoft/LightGBM/actions/runs/12383890420/job/34567465751#step:2:22)78F.

This is a well known issue without any proper solution:

• Secondary rate limit dessant/lock-threads#50
• You have exceeded a secondary rate limit. dessant/lock-threads#48
• Rate limiting dessant/lock-threads#35

Syntax for process-only argument was taken from here: https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#schedule

A single workflow can be triggered by multiple schedule events. You can access the schedule event that triggered the workflow through the github.event.schedule context. This example triggers the workflow to run at 5:30 UTC every Monday-Thursday, but skips the Not on Monday or Wednesday step on Monday and Wednesday.

This PR proposes splitting issues and PRs into separate workflow runs. Hopefully, it should decrease the number of performed requests at once and help to avoid exhausting requests quota.

[borchero]

Let's try 😅