Replace set-ouput with environment files

GitHub Actions has deprecated the use of the `set-output` command [[1]],
we should use environment files instead.

[1]: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Author: lfdebrux

.github/workflows/publish.yaml

@@ -24,14 +24,14 @@ jobs:
         id: gem_version
         run: |
           gem_version=$(ruby -r rubygems -e "puts Gem::Specification::load('govuk_tech_docs.gemspec').version")
-          echo "::set-output name=gem_version::$gem_version"
+          echo "gem_version=$gem_version" >> "$GITHUB_OUTPUT"
 
           if git fetch origin "refs/tags/v$gem_version" >/dev/null 2>&1
           then
             echo "Tag 'v$gem_version' already exists"
-            echo "::set-output name=new_version::false"
+            echo "new_version=false" >> "$GITHUB_OUTPUT"
           else
-            echo "::set-output name=new_version::true"
+            echo "new_version=true" >> "$GITHUB_OUTPUT"
           fi
 
   deploy: