Update CI matrix

[Mangara]

Rails 7.2 was released a little over a month ago, so it should be included in the CI matrix.
Now that Rails edge is Rails 8.0, it dropped support for Ruby 3.1, so I excluded that.

This is the current matrix: (+ means we test it):

Rails \ Ruby	2.6	2.7	3.0	3.1	3.2	3.3
Rails 5.2	+	+	-	-	-	-
Rails 6.0	+	+	+	-	-	-
Rails 6.1	+	+	+	+	-	-
Rails 7.0	-	+	+	+	+	+
Rails 7.1	-	-	-	+	+	+
Rails 7.2	-	-	-	+	+	+
Rails edge	-	-	-	-	+	+