(maint) Remove Rubygems pin

In 52b0dee, we pinned Rubygems to 3.3.26 for Ruby 2.5 compatibility. In
the Puppet 8 series where the minimum supported version of Ruby is 3.1,
we no longer need to pin to that version of Rubygems.

This commit removes the pin to Rubygems 3.3.26 in the Checks and Rspec
tests GitHub Actions workflows.

Author: mhashizume

.github/workflows/checks.yaml

@@ -34,7 +34,7 @@ jobs:
 
       - name: Update rubygems and install gems
         run: |
-          gem update --system 3.3.26 --silent --no-document
+          gem update --system --silent --no-document
           bundle config set without packaging documentation
           bundle install --jobs 4 --retry 3
 

.github/workflows/rspec_tests.yaml

@@ -35,7 +35,7 @@ jobs:
 
       - name: Update rubygems and install gems
         run: |
-          gem update --system 3.3.26 --silent --no-document
+          gem update --system --silent --no-document
           bundle config set without packaging documentation
           bundle install --jobs 4 --retry 3