Prevent Dependabot PRs for Minitest 6.x

Update configuration to avoid Minitest 6.x updates on Rails 7.x
gemfiles, which are still tested with Ruby 3.1.

This prevents Dependabot from opening PRs incompatible with the
supported Ruby versions, as happened with #983, #984, and #985.

Author: tagliala

.github/dependabot.yml

@@ -27,6 +27,8 @@ updates:
         patterns:
           - "*"
     ignore:
+      - dependency-name: minitest
+        versions: ">= 6.0.0"
       - dependency-name: rails
         versions: ">= 7.1.0"
       - dependency-name: railties
@@ -43,6 +45,8 @@ updates:
     ignore:
       - dependency-name: erb
         versions: ">= 5"
+      - dependency-name: minitest
+        versions: ">= 6.0.0"
       - dependency-name: rails
         versions: ">= 7.2.0"
       - dependency-name: railties
@@ -59,6 +63,8 @@ updates:
     ignore:
       - dependency-name: erb
         versions: ">= 5"
+      - dependency-name: minitest
+        versions: ">= 6.0.0"
       - dependency-name: rails
         versions: ">= 8.0.0"
       - dependency-name: railties