Automatically add dependabot label to dependabot PRs

[cwperks]

Description

The dependabot-changelog-helper GHA expects the dependabot label on dependabot PRs. Currently the Update the changelog step in dependabot_pr.yml workflow is failing silently bc this label is absent

Run dangoslen/dependabot-changelog-helper@v4
Label 'dependabot' not found in pull request labels

ref: https://github.com/opensearch-project/security/actions/runs/15109843244/job/42484278169?pr=5345

• Category (Enhancement, New feature, Bug fix, Test fix, Refactoring, Maintenance, Documentation)

Maintenance

Check List

• New functionality includes testing
• New functionality has been documented
• New Roles/Permissions have a corresponding security dashboards plugin PR
• API changes companion pull request created
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.78%. Comparing base (d7435f0) to head (282f708).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5346      +/-   ##
==========================================
- Coverage   71.90%   71.78%   -0.12%     
==========================================
  Files         378      378              
  Lines       23562    23562              
  Branches     3630     3630              
==========================================
- Hits        16942    16914      -28     
- Misses       4810     4841      +31     
+ Partials     1810     1807       -3     

see 12 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.