#### What type of PR is this?
/kind cleanup
/area core
#### What this PR does / why we need it:
This PR bumps all version to 2.20.0 to prepare for next development.
#### Does this PR introduce a user-facing change?
```release-note
None
```
#### What type of PR is this?
/kind cleanup
/area core
#### What this PR does / why we need it:
This is a regular updates after a new release.
#### Does this PR introduce a user-facing change?
```release-note
None
```
#### What type of PR is this?
/kind cleanup
/area core
#### What this PR does / why we need it:
Regenerate API docs and client to prevent unrelated changes from other PRs.
#### Does this PR introduce a user-facing change?
```release-note
None
```
#### What type of PR is this?
/kind cleanup
/area core
#### What this PR does / why we need it:
This PR adds support for generating API docs into project and generate API client according the API docs.
To generate/update latest API docs, execute the following command:
```bash
./gradlew clean generateOpenApiDocs
```
To generate/update latest API client, execute the following command:
```bash
make -C ui api-client-gen
```
Meanwhile, I also remove the lint on API client due to unnecessary.
Supersedes of https://github.com/halo-dev/halo/pull/5637
#### Does this PR introduce a user-facing change?
```release-note
None
```
John Niang