Revert "Automate the update of versions.json"

What does this MR do and why?

Reverts !3706 (merged).

On the first deploy after 00:00 GMT on May 1, the rspec test for the automated versions dropdown method began to fail, breaking the pipeline. The versions dropdown function began returning one version ahead of where it should:

Failures:
  1) Release#update_versions_dropdown when not in DRY_RUN mode updates content/versions.json
     Failure/Error: File.write('content/versions.json', JSON.pretty_generate(dropdown_json))
       #<File (class)> received :write with unexpected arguments
         expected: ("content/versions.json", "[\n  {\n    \"next\": \"16.0\",\n    \"current\": \"15.11\",\n    \"last_minor\": [\n      \"15.10\"...",\n      \"15.9\"\n    ],\n    \"last_major\": [\n      \"14.10\",\n      \"13.12\"\n    ]\n  }\n]")
              got: ("content/versions.json", "[\n  {\n    \"next\": \"16.1\",\n    \"current\": \"16.0\",\n    \"last_minor\": [\n      \"15.11\",...,\n      \"15.10\"\n    ],\n    \"last_major\": [\n      \"14.10\",\n      \"13.12\"\n    ]\n  }\n]")

This is likely a date-handling problem with code introduced in !3706 (merged). Seems to be using the first of the month instead of the 22nd for deciding what the next release is.

As this is not used for the website itself, we should be able to just revert this for now and continue manually updating versions.json as part of the release.

Reverting this MR also closes #1618 (closed).

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