Remove 6.0.x from Dependabot config

Closes gh-14227

.github/dependabot.yml

@@ -49,26 +49,6 @@ updates:
       - dependency-name: "*"
         update-types: [ "version-update:semver-major", "version-update:semver-minor" ]
 
-  - package-ecosystem: "gradle"
-    target-branch: "6.0.x"
-    milestone: 143 # 6.0.x
-    directory: "/"
-    schedule:
-      interval: "daily"
-      time: "03:00"
-      timezone: "Etc/UTC"
-    labels: [ "type: dependency-upgrade" ]
-    ignore:
-      - dependency-name: "com.nimbusds:nimbus-jose-jwt" # nimbus-jose-jwt gets updated when oauth2-oidc-sdk is updated to ensure consistency
-      - dependency-name: "org.python:jython" # jython updates break integration tests
-      - dependency-name: "org.apache.directory.server:*" # ApacheDS version > 1.5.5 contains break changes
-      - dependency-name: "org.junit:junit-bom"
-        update-types: [ "version-update:semver-major" ]
-      - dependency-name: "org.mockito:mockito-bom"
-        update-types: [ "version-update:semver-major" ]
-      - dependency-name: "*"
-        update-types: [ "version-update:semver-major", "version-update:semver-minor" ]
-
   - package-ecosystem: "gradle"
     target-branch: "5.8.x"
     milestone: 246 # 5.8.x