#### What type of PR is this?
/kind bug
/area core
/milestone 2.21.x
#### What this PR does / why we need it:
This PR replaces the dependency `thymeleaf-spring6` to apply the memory leak fix of <https://github.com/halo-dev/thymeleaf/pull/1>.
Please note that this is a temporary fix, and we would revert this PR after Thymeleaf official released a new version including my fix.
#### Which issue(s) this PR fixes:
Fixes https://github.com/halo-dev/halo/issues/7289
#### Special notes for your reviewer:
1. Execute command ab -c 100 -n 1000 -H 'Accept: text/html' -H 'Cache-Control: no-cache' http://localhost:8090/ and then press Ctrl + C to stop the thread.
2. See logs in Halo
#### Does this PR introduce a user-facing change?
```release-note
修复在特定情况下模板引擎可能出现内存泄漏的问题
```
John Niang