Windows update license documentation invokes deprecated/removed '_xpack' path

[danielharada]

Elasticsearch version (bin/elasticsearch --version):
7.6

OS version (uname -a if on a Unix-like system):
Windows

Description of the problem including expected versus actual behavior:
Our Update License API docs for Windows lists the following endpoint for updating the license: http://<host>:<port>/_xpack/license.

However, the _xpack path was deprecated, and likely removed by version 7.6: #35959. This path was (erroneously?) added back into the docs for Windows only in a subsequent PR: #40375.

Attempts to update license at the documented endpoint fail, but removing the _xpack path succeeds: http://<host>:<port>/license.

Steps to reproduce:

Please include a minimal but complete recreation of the problem, including
(e.g.) index creation, mappings, settings, query etc. The easier you make for
us to reproduce it, the more likely that somebody will take the time to look at it.

1. Setup ES on Windows
2. Get a valid license
3. PUT your license at http://<host>:<port>/_xpack/license
   Results
   License not updated
4. PUT your license at http://<host>:<port>/license
   Results
   200 OK response and license successfully updated

Provide logs (if relevant):

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[elasticmachine]

Pinging @elastic/es-security (:Security/License)