chore: remove dependabot in favor of weekly batch dependency updates [RED-695] (#1395)

Dependabot opened up to 3 npm version-update PRs daily, duplicating the weekly
batched dependency update that is Node-floor aware and respects the
minimumReleaseAge embargo in pnpm-workspace.yaml. Its output was already being
discarded: the five most recent Dependabot PRs were closed rather than merged.

Dependabot's version updates were the only remaining producer of the
`dependencies` label, so the `Improvements` category that mapped it in
.github/release.yml is removed too, along with the `dependencies` exclusion
under `Other Changes`. Weekly batch PRs are unlabeled and already fall into
`Other Changes`, so generated release notes are unaffected in practice.

Dependabot security alerts and security updates are a repository setting rather
than a function of this config and remain enabled. Should a security-update PR
be opened and merged, it carries the `dependencies` label and is now grouped
under `Other Changes`.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: sorccu

.github/dependabot.yml

@@ -12,9 +12,3 @@ changelog:
     - title: Other Changes
       labels:
         - '*'
-      exclude:
-        labels:
-          - dependencies
-    - title: Improvements
-      labels:
-        - "dependencies"