[DOCS] Replace hard-coded links to master

[lcawl]

Relates to elastic/docs#3160

Description

This PR updates links that target the master branch of the documentation, for example links in https://www.elastic.co/guide/en/search-ui/current/api-connectors-elasticsearch.html

The are causing the following documentation build errors when we retire the master documentation:

...
INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/search-ui/current/api-connectors-elasticsearch.html contains broken links to:
--
  | INFO:build_docs:   - en/kibana/master/api-keys.html

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