Changes

opt(prow-jobs): skip triggering CI jobs for changes on git ignore file (#3515)

This pull request makes changes to the `skip_if_only_changed`
configuration across multiple Prow job configuration files. The primary
update adds `.gitignore` files to the list of file types that, when
modified, will not trigger specific CI jobs. Additionally, the changes
introduce YAML anchors and references to reduce redundancy in some
configurations.

### Updates to `skip_if_only_changed`:

* Added `.gitignore` to the `skip_if_only_changed` regex pattern in
multiple files, including `prow-jobs/pingcap-inc/tici/presubmits.yaml`,
`prow-jobs/pingcap/community/presubmits.yaml`, and others. This ensures
that changes to `.gitignore` files do not trigger unnecessary CI jobs.
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### Configuration Simplification:

* Introduced YAML anchors (`&brancher` and `&skip_if_only_changed`) and
references (`*brancher`, `*skip_if_only_changed`) in
`prow-jobs/pingcap/tidb/latest-postsubmits.yaml` to avoid duplication of
branch definitions and `skip_if_only_changed` logic across multiple job
entries.
 (commit: 87f922a)