Fix GHA release flow

[ronaldtse]

Due to namespacing issues it's not easy to get it to work:

1. The release GHA workflow looks for the correct namespace according to gem name to find the VERSION constant. This means it expects the VERSION constant to be in lib/jekyll/theme/rop/version.rb, but we cannot break Jekyll::Theme which is a class.
2. Jekyll uses zeitwerk which depends on the source code file name/paths to reflect the actual class hierarchy. This means if we use Jekyll::Theme::Rop::VERSION then it expects all other code to also be in lib/jekyll/theme/rop/* and hence under the Jekyll::Theme::Rop namespace.

[ronaldtse]

We won't have this problem anymore since we will rename the gem as per #88 . Closing.