CI: Bump actions/checkout to v6 (#56)

Author: pilaf

.github/workflows/ci.yml

@@ -41,7 +41,7 @@ jobs:
     name: Test against Ruby ${{ matrix.ruby }} / ActiveModel ${{ matrix.activerecord }}
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:

.gitignore

@@ -21,6 +21,7 @@ tmp
 *.a
 mkmf.log
 spec/db/*
+.tool-versions
 
 # rspec failure tracking
 .rspec_status