Add re2 2023-07-01 to the CI build matrix

As re2 2023-07-01 requires Abseil, we use Rob Savoury's Build Tools PPA
to install it on Ubuntu 20.04. As the legacy Ruby builds run on Ubuntu
16.04 and the version of cmake there is too old to compile Abseil, we
skip newer ABI versions of re2 for Ruby 2.0 and older.

There are fatal problems with Bundler and older Ruby versions on Ubuntu
22.04 (see ruby/setup-ruby#496) otherwise we
could benefit from libabsl-dev being available without a PPA.

.github/workflows/tests.yml

@@ -36,10 +36,17 @@ jobs:
             soname: 9
           - version: "20221201"
             soname: 10
+          - version: "20230701"
+            soname: 11
     steps:
       - uses: actions/checkout@v3
       - name: Remove any existing libre2 installation
         run: sudo apt-get remove -y libre2-dev libre2-5
+      - name: Install Abseil for newer re2 releases
+        run: |
+          sudo apt-get install -y software-properties-common
+          sudo add-apt-repository ppa:savoury1/build-tools
+          sudo apt-get install -y libabsl-dev
       - name: Download and install specific release of libre2
         run: |
           curl -Lo libre2-dev.deb https://github.com/mudge/re2-ci/releases/download/v2/libre2-dev_${{ matrix.libre2.version }}_amd64.deb