ci: Disable pull request trigger for update changelogs workflow (#8673)

## Explanation

This workflow always fails with this error message:

> Determine whether this PR is a release PR
> Branch "refs/pull/8672/merge" is not allowed to deploy to
default-branch due to environment protection rules.

To avoid confusion, I just removed the pull request trigger for now,
until we figure out how to fix this.

## References

<!--
Are there any issues that this pull request is tied to?
Are there other links that reviewers should consult to understand these
changes better?
Are there client or consumer pull requests to adopt any breaking
changes?

For example:

* Fixes #12345
* Related to #67890
-->

## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/processes/updating-changelogs.md)
- [ ] I've introduced [breaking
changes](https://github.com/MetaMask/core/tree/main/docs/processes/breaking-changes.md)
in this PR and have prepared draft pull requests for clients and
consumer packages to resolve them

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: this only changes when the workflow runs, not its execution
logic. The main impact is reduced automatic coverage since PR-open
events will no longer trigger changelog updates.
> 
> **Overview**
> **Stops `Update Changelogs` from running on PR open.** The workflow’s
`pull_request` trigger is commented out, leaving it to run only on
`issue_comment` creation (e.g., `@metamaskbot update-changelogs`),
avoiding failures caused by environment protection rules on PR merge
refs.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
ee3cd62. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: Mrtenz

.github/workflows/update-changelogs.yml

@@ -4,11 +4,11 @@ on:
   issue_comment:
     types:
       - created
-  pull_request:
-    branches:
-      - main
-    types:
-      - opened
+#  pull_request:
+#    branches:
+#      - main
+#    types:
+#      - opened
 
 permissions:
   contents: write