Changes

feature(prow-jobs/pingcap/tidb): enable auto run for pull_build_next_gen job (#3602)

This pull request updates the `context` fields in the
`latest-presubmits-next-gen.yaml` file to reflect non-blocking statuses
for various presubmit jobs. Additionally, it removes commented-out lines
and ensures proper configuration for `skip_if_only_changed` and
`run_if_changed` fields.

### Updates to presubmit job configurations:

* **Context field updates**:
- Changed `context` values to indicate non-blocking statuses for the
following jobs:
    - `pull-build-next-gen` → `non-block-pull-build-next-gen`
    - `pull-check-next-gen` → `non-block-pull-check-next-gen`
    - `pull-unit-test-next-gen` → `non-block-pull-unit-test-next-gen`
- `pull-unit-test-ddlv1-next-gen` →
`non-block-pull-unit-test-ddlv1-next-gen`
- `pull-mysql-client-test-next-gen` →
`non-block-pull-mysql-client-test-next-gen`

* **Configuration fixes**:
- Removed commented-out `skip_if_only_changed` lines and replaced them
with active configurations for relevant jobs.
[[1]](diffhunk://#diff-6d136e7cf0b5751935f337e8b7065f23f4f547f7f85072637dbab6476832439eL15-R27)
[[2]](diffhunk://#diff-6d136e7cf0b5751935f337e8b7065f23f4f547f7f85072637dbab6476832439eL55-R67)
- Enabled the `run_if_changed` field for `pull-unit-test-ddlv1-next-gen`
with the pattern `"pkg/(ddl|meta)/.*"`.
 (commit: 9af9d84)