ci: update workflow

[Phoenix500526]

Please briefly answer these questions: Currently, our depend-bot update is a little bit frequent, therefore I reduce the open-pull-requests-limit from 10 to 3 per week.

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

  1. reduce the open-pull-requests-limit from 10 to 3 per week.
  2. update the correct release slash-assign-action

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[mergify[bot]]

@Phoenix500526 You've modified the workflows. Please don't forget to update the .mergify.yml.

[codecov[bot]]

Codecov Report

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

Project coverage is 75.58%. Comparing base (e35b35a) to head (39eafc6). Report is 81 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #817 +/- ## ========================================== + Coverage 75.55% 75.58% +0.03% ========================================== Files 180 186 +6 Lines 26938 27472 +534 Branches 26938 27472 +534 ========================================== + Hits 20353 20765 +412 - Misses 5366 5430 +64 - Partials 1219 1277 +58 ```

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