REST endpoints xpack.license.post_start_basic|trial should be renamed

[clintongormley]

The REST endpoints xpack.license.post_start_basic and xpack.license.post_start_trial should be renamed to xpack.license.start_basic and xpack.license.start_trial respectively, to be consistent with other endpoints.

/cc @elastic/es-clients

[elasticmachine]

Pinging @elastic/es-security

[Mpdreamz]

Much better names indeed! For awareness as of #39063 you can now deprecate an API an introduce the renamed one rather then simply renaming the spec file.

This allows the clients to not break already shipped code.