Fix warnings issued from successive convert_units_to calls

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR addresses concerns raised in review of Remove deprecated indicators/indices, handle warnings  #1318
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGES.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Fixes a bug in the logic of two indices (snowfall_approximation with method="brown", effective_growing_degree_days) that was emitting usage warnings from valid calls.
• prepends v to entries in CHANGES.rst to properly generate hyperlink entries.
• Warnings from set _version_deprecated within Indicators now emit FutureWarning instead of DeprecationWarning for greater visibility.

Does this PR introduce a breaking change?

No.

Other information:

See also: #1010