Add a .ruby-version file

In two recent pull requests, new contributors have had CI failing
because they used Ruby 3.3 (or older) to run the documentation Rake
tasks. (Ruby 3.4 formats hashes a bit differently, adding whitespace
around hashrockets.)

By adding a .ruby-version file, most contributors will automatically be
using the same Ruby version. Ideally we would not need to specify the
patch version in .ruby-version, but I believe that the
`major.minor.patch` format is what is compatible with the most Ruby
version managers out there.

I found this discussion about writing a specification for .ruby-version,
but it doesn't seem to be completed yet:
ruby/rubygems#7074

Author: bquorning

.github/workflows/main.yml

@@ -18,7 +18,6 @@ jobs:
       - uses: actions/checkout@v5
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.4"
           bundler-cache: true
       - run: bundle exec rake confirm_config documentation_syntax_check confirm_documentation
 
@@ -54,7 +53,6 @@ jobs:
       - uses: actions/checkout@v5
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.4"
           bundler-cache: true
       - run: bundle exec rake spec
 
@@ -74,7 +72,6 @@ jobs:
           cat Gemfile.local
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.4"
           bundler-cache: true
       - name: Show RuboCop version
         run: grep '^    rubocop' Gemfile.lock | sort
@@ -96,7 +93,6 @@ jobs:
           cat Gemfile.local
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.4"
           bundler-cache: true
       - name: Show RuboCop version
         run: grep '^    rubocop' Gemfile.lock | sort
@@ -119,7 +115,6 @@ jobs:
           EOF
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.4"
           bundler-cache: true
       - run: NO_COVERAGE=true bundle exec rake spec
 

.gitignore

@@ -16,7 +16,6 @@ pkg
 /vendor
 
 .ruby-gemset
-.ruby-version
 
 # simplecov generated
 coverage

.ruby-version

@@ -0,0 +1 @@
+3.4.7