Restore support for Ruby 2.6 and 2.7

Upgrade to rake-compiler-dock 1.8.0 which allows us to compile Musl gems
for Ruby 3.0 and restore support for Ruby 2.6 and 2.7 at the same time.
This requires installing the latest supported version of RubyGems for
older versions of Ruby.

We now switch to Ruby 3.4 final (rather than using the release
candidate) on all platforms for both precompilation and testing.

.github/workflows/dependencies.yml

@@ -12,7 +12,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.4.0-rc1"
+          ruby-version: "3.4"
       - name: Upgrade all vendored dependencies to their latest versions
         run: ./scripts/update-dependencies
       - uses: actions/create-github-app-token@v1

.github/workflows/docs.yml

@@ -13,7 +13,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.4.0-rc1"
+          ruby-version: "3.4"
       - run: gem install --no-document yard redcarpet
       - run: yardoc
       - uses: actions/upload-pages-artifact@v3

.github/workflows/precompile-gem.yml

@@ -14,7 +14,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.4.0-rc1"
+          ruby-version: "3.4"
           bundler-cache: true
       - uses: actions/cache@v4
         with: