#### What type of PR is this?
/kind cleanup
/area core
#### What this PR does / why we need it:
Upgrade to Gradle 8.0.2 by using command `./gradlew wrapper --gradle-version 8.0.2 --distribution-type bin`.
Recently, we have a plan to restructure our project via Gradle 8, so I upgrade to Gradle 8.0.2 now.
#### Does this PR introduce a user-facing change?
```release-note
None
```
* Update gradle wrapper version
* Update spring boot version to 2.5.0-M2
* Fix wrong const of temp_dir
* Refactor error controller
* Fix startup error due to theme not found
* Refine error controller handler
* Refine multipart resolver config
* Fix ThemeRepositoryImplTest error
* Fix freemarker not found error
* chore: change jetty to undertow.
* Remove useless throws
Co-authored-by: Ryan Wang <i@ryanc.cc>
* Update gradle wrapper version to 6.6.1
* Upgrade h2 version to 1.4.197
* Make controller log more details
* Refactor FileHandler
* Fix image reader error