Add deprecation notice to source map upload docs, redirect to Kibana API

[axw]

We are deprecating source map upload via apm-server in 7.16, and removing it in 8.0. We need to add a deprecation notice to the docs, e.g. https://www.elastic.co/guide/en/apm/server/current/sourcemap-api.html, and direct users to https://www.elastic.co/guide/en/kibana/current/rum-sourcemap-api.html.

Note that the new Kibana API currently only works with the Fleet integration. We intend for standalone to work with this in 8.0, tracked in #5993.