Bump peternied/discerning-merger from 2 to 3 (opensearch-project#3576)

### Description
Bumps
[peternied/discerning-merger](https://github.com/peternied/discerning-merger)
from 2 to 3.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/peternied/discerning-merger/releases">peternied/discerning-merger's
releases</a>.</em></p>
<blockquote>
<h2>v3 Release</h2>
<ul>
<li>Allows specifying how the merge is performed, with default of
'squash' method</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/peternied/discerning-merger/commit/07eafb14c195a2c23b291f6b008a686589cfb545"><code>07eafb1</code></a>
Support passing merge type</li>
<li>See full diff in <a
href="https://github.com/peternied/discerning-merger/compare/v2...v3">compare
view</a></li>
</ul>
</details>

### Check List
- [ ] ~New functionality includes testing~
- [ ] ~New functionality has been documented~
- [X] 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](https://github.com/opensearch-project/OpenSearch/blob/main/CONTRIBUTING.md#developer-certificate-of-origin).

Signed-off-by: Peter Nied <[email protected]>

.github/workflows/automatic-merges.yml

@@ -19,7 +19,7 @@ jobs:
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
 
-      - uses: peternied/discerning-merger@v2
+      - uses: peternied/discerning-merger@v3
         if: steps.find-triggering-pr.outputs.pr-number != null
         with:
           token: ${{ secrets.GITHUB_TOKEN }}