CI: Drop Rails 7.0 but add Rails 8.0 builds

Rails 7.0 is untestable because the rails _7.0_ new command
fails because of a bug that has not been fixed in the published
rails gem and wont be ever. Since rails 7.0 will be eol in 6 weeks
it does not make sense to waste carbon on it.

Author: tvdeyen

.github/workflows/test.yml

@@ -23,35 +23,30 @@ jobs:
       fail-fast: true
       matrix:
         rails-version:
-          - "7.0"
           - "7.1"
           - "7.2"
+          - "8.0"
         ruby-version:
           - "3.1"
           - "3.4"
         solidus-branch:
-          - "v4.1"
-          - "v4.2"
           - "v4.3"
           - "v4.4"
           - "v4.5"
+          - "main"
         database:
           - "postgresql"
           - "mysql"
           - "sqlite"
         exclude:
           - rails-version: "7.2"
             solidus-branch: "v4.3"
-          - rails-version: "7.2"
-            solidus-branch: "v4.2"
-          - rails-version: "7.2"
-            solidus-branch: "v4.1"
-          - rails-version: "7.1"
-            solidus-branch: "v4.2"
-          - rails-version: "7.1"
-            solidus-branch: "v4.1"
-          - ruby-version: "3.4"
-            rails-version: "7.0"
+          - ruby-version: "3.1"
+            rails-version: "8.0"
+          - solidus-branch: "v4.3"
+            rails-version: "8.0"
+          - solidus-branch: "v4.4"
+            rails-version: "8.0"
     steps:
       - uses: actions/checkout@v4
       - name: Run extension tests