✅ Exclude incompatible versions from CI matrix

With a little effort, some of these could be made compatible, but...
it's really not worth the effort.  We'll be dropping support for older
versions from the next minor release anyway.

.github/workflows/ruby.yml

@@ -35,6 +35,19 @@ jobs:
             resque: "2.6"
             redis: "5.2"
 
+        exclude:
+          - { resque: "2.5", ruby: "2.4" } # redis 5.0 requires ruby 2.5
+          - { resque: "2.6", ruby: "2.4" } # redis 5.2 requires ruby 2.6
+          - { resque: "2.6", ruby: "2.5" } # redis 5.2 requires ruby 2.6
+
+          # ironically, resque 1.22 seems to be compatible with ruby 3+
+          - { resque: "1.23", ruby: "3.0" } # incompatible with ruby 3+
+          - { resque: "1.24", ruby: "3.1" } # incompatible with ruby 3+
+          - { resque: "1.25", ruby: "3.2" } # incompatible with ruby 3+
+
+          - { resque: "2.1", ruby: "3.0" } # redis-namespace 1.7 requires ruby < 3
+          - { resque: "2.1", ruby: "3.1" } # redis-namespace 1.7 requires ruby < 3
+          - { resque: "2.1", ruby: "3.2" } # redis-namespace 1.7 requires ruby < 3
 
     services:
       redis: