[DOCS] Fixes broken link to sliced scroll

[lcawl]

Related to elastic/docs#1932

This PR fixes the following links that will break when the "current" documentation branch becomes 7.9:

13:54:52 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/elasticsearch/client/net-api/7.x/scrolling-documents.html:
13:54:52 INFO:build_docs:   - en/elasticsearch/reference/current/search-request-body.html#sliced-scroll
13:54:52 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/elasticsearch/client/net-api/current/scrolling-documents.html:
13:54:52 INFO:build_docs:   - en/elasticsearch/reference/current/search-request-body.html#sliced-scroll
13:54:52 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/elasticsearch/client/net-api/master/scrolling-documents.html:
13:54:52 INFO:build_docs:   - en/elasticsearch/reference/current/search-request-body.html#sliced-scroll

NOTE: This fixes changes the link to https://www.elastic.co/guide/en/elasticsearch/reference/7.9/paginate-search-results.html. It does not change the link to https://www.elastic.co/guide/en/elasticsearch/reference/7.9/paginate-search-results.html#slice-scroll since that section ID does not exist until current=7.9. If necessary, we can update the URL again after 7.9 is released.

[russcam]

LGTM, thanks @lcawl!

[russcam]

Added backport labels for 7.x and 7.9 which will trigger github actions to auto open PRs for those branches when this PR is merged in.