Align Dependabot PRs with CONTRIBUTING

Previously Dependabot was setup to submit PRs to every branch.
However, this does not align with the contributing guidelines which
state to only submit a PR on the oldest branch so that merge forward
strategy can be used.

This changes the dependabot configuration to better align with our
contributing guidelines:

- PRs for github actions are submitted against the oldest branch since
  all branches will need updated using a merge forward stategy. Merging a
  github action will require us to merge forward manually and preserve
  the changes in the oldest branch to pickup the github actions update.
- Java dependencieds are submitted against each branch since they will
  need to merge -s ours to preserve the correct major.minor semantics.
  Merging a java dependency will now require us to do the merging manually.

Author: rwinch

.github/dependabot.yml

@@ -86,16 +86,6 @@ updates:
         update-types:
           - version-update:semver-major
 
-  - package-ecosystem: github-actions
-    target-branch: 6.4.x
-    directory: /
-    schedule:
-      interval: weekly
-    labels:
-      - 'type: task'
-      - 'in: build'
-    ignore:
-      - dependency-name: sjohnr/*
   - package-ecosystem: github-actions
     target-branch: 6.3.x
     directory: /
@@ -106,14 +96,6 @@ updates:
       - 'in: build'
     ignore:
       - dependency-name: sjohnr/*
-  - package-ecosystem: github-actions
-    target-branch: main
-    directory: /
-    schedule:
-      interval: weekly
-    labels:
-      - 'type: task'
-      - 'in: build'
   - package-ecosystem: github-actions
     target-branch: docs-build
     directory: /