(#21427) make supported_formats return an array of symbols

Prior to this commit, supported_formats was returning an array of
strings, which violated the contract for FormatHandler's
most_suitable_format_for method.  This commit fixes supported_formats
to return an array of symbols.
Merge pull request #1874 from kylog/issue/21427-fixup

(#21427) make supported_formats return an array of symbols
Merge remote-tracking branch 'upstream/stable'

* upstream/stable:
  (#21427) make supported_formats return an array of symbols
Merge branch 'master' of https://github.com/puppetlabs/puppet