Changes

feat(pingcap/tidb): update presubmits for next-gen branches and context names (#3664)

Add support for release-nextgen branches and make pull-build-next-gen
and pull-check-next-gen contexts required instead of optional.
---
This pull request updates the
`prow-jobs/pingcap/tidb/latest-presubmits-next-gen.yaml` file to enhance
branch targeting and modify presubmit job configurations for better
alignment with the new "next-gen" workflow. The key changes include
adding support for new branch patterns and updating job contexts to
reflect mandatory checks.

### Branch targeting updates:
* Added support for branches matching the pattern `^release-nextgen-\d+
Add support for release-nextgen branches and make pull-build-next-gen
and pull-check-next-gen contexts required instead of optional.

in the global `brancher` configuration, enabling the inclusion of
"next-gen" release branches.
(`[prow-jobs/pingcap/tidb/latest-presubmits-next-gen.yamlR5](diffhunk://#diff-6d136e7cf0b5751935f337e8b7065f23f4f547f7f85072637dbab6476832439eR5)`)

### Presubmit job configuration updates:
* Updated the `context` for the `pull-build-next-gen` job to remove the
"non-block" prefix, making it a required check.
(`[prow-jobs/pingcap/tidb/latest-presubmits-next-gen.yamlL16-R17](diffhunk://#diff-6d136e7cf0b5751935f337e8b7065f23f4f547f7f85072637dbab6476832439eL16-R17)`)
* Updated the `context` for the `pull-check-next-gen` job to remove the
"non-block" prefix, also making it a required check.
(`[prow-jobs/pingcap/tidb/latest-presubmits-next-gen.yamlL26-R26](diffhunk://#diff-6d136e7cf0b5751935f337e8b7065f23f4f547f7f85072637dbab6476832439eL26-R26)`)
 (commit: 4d3ed5b)