Stop passing disable-error_highlight

The `error_highlight` gem was added in Ruby 3.1, which means we cannot
disable it on Ruby 3.0 or earlier.

So long as we support those versions, let's stop disabling it. This is
the fastest way to get CI passing again.

.github/workflows/build.yml

@@ -80,7 +80,6 @@ jobs:
 
     env:
       BUNDLE_GEMFILE: gemfiles/${{ matrix.rails }}.gemfile
-      RUBYOPT: "--disable-error_highlight"
 
     steps:
       - uses: actions/checkout@v4