Update DOCS_BRANCH from 8.x to 8.19

[lcawl]

This PR updates the DOCS_BRANCHi in the Makefile so that the appropriate URLs are generated.
I noticed it was still linking to "8.x" URLs in #5339

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[pquentin]

Thanks! LGTM.

Note that the 8.19 branch still references 8.x in the overlays and some spec files.

docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/es-connectors-tutorial-api.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/data-streams.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/docs-replication.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/eql.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/esql.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/kibana/8.x/xpack-graph.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/index-lifecycle-management.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/logstash/8.x/logstash-centralized-pipeline-management.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/machine-learning/8.x/ml-ad-finding-anomalies.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/machine-learning/8.x/ml-dfa-overview.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/machine-learning/8.x/ml-nlp-overview.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/query-dsl-rule-query.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/modules-scripting.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/snapshot-restore.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/snapshots-take-snapshot.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/xpack-sql.html
docs/overlays/elasticsearch-shared-overlays.yaml:            url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/xpack-alerting.html
docs/overlays/elasticsearch-shared-overlays.yaml:        url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/field-and-document-access-control.html#templating-role-query
docs/overlays/elasticsearch-shared-overlays.yaml:          url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/query-dsl.html
docs/overlays/elasticsearch-shared-overlays.yaml:          url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/analysis-tokenizers.html
docs/overlays/elasticsearch-shared-overlays.yaml:          url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/query-dsl.html
docs/overlays/elasticsearch-shared-overlays.yaml:          url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/query-dsl.html
docs/overlays/elasticsearch-shared-overlays.yaml:        url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/collapse-search-results.html
docs/overlays/elasticsearch-shared-overlays.yaml:        url: https://www.elastic.co/guide/en/elasticsearch/reference/8.x/index-modules.html
specification/ilm/_types/Phase.ts:   * The freeze action is a noop in 8.x
specification/license/get/GetLicenseRequest.ts:     * This parameter is deprecated and will always be set to true in 8.x.

Should we update at least the overlays in this PR?

[lcawl]

Should we update at least the overlays in this PR?

Good catch! I've updated the URLs in the overlay in 3f7648d