Add default_auto_field to BlogAppConfig

[protoroto]

Description

Add default_auto_field to BlogAppConfig to get rid of Django 3.2 warnings

References

Fix #737

Checklist

• [x] I have read the contribution guide
• [x] Code lint checked via inv lint
• [x] changes file included (see docs)
• [ ] Usage documentation added in case of new features
• [ ] Tests added

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (bc43947) 94.24% compared to head (14b80ba) 94.24%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #741 +/- ## ======================================== Coverage 94.24% 94.24% ======================================== Files 26 26 Lines 1823 1824 +1 Branches 260 260 ======================================== + Hits 1718 1719 +1 Misses 53 53 Partials 52 52 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `94.24% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nephila#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/nephila/djangocms-blog/pull/741?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nephila) | Coverage Δ | | |---|---|---| | [djangocms\_blog/apps.py](https://app.codecov.io/gh/nephila/djangocms-blog/pull/741?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nephila#diff-ZGphbmdvY21zX2Jsb2cvYXBwcy5weQ==) | `100.00% <100.00%> (ø)` | |

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[coveralls]

coverage: 97.094% (+0.1%) from 96.961% when pulling 14b80ba27c8ef7f9ef3e6a93de7fa7013d709fb7 on protoroto:feature/issue-737-add-default-auto-field into bc43947bdaf3fd38d588ba9e9323ff501944d2d0 on nephila:develop.

[yakky]

:+1: