Remove reference to `@dependabot merge` from PR description

[jfrost-mo]

Is there an existing issue for this?

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What you expected to see, versus what you actually saw

Several dependabot commands have been removed, as detailed here: https://github.blog/changelog/2026-01-27-changes-to-github-dependabot-pull-request-comment-commands/

However, these commands are still suggested in the pull request description of pull requests opened by dependabot, such as MetOffice/CSET#1893.

The dependabot pull request description should be updated to remove these commands, so people don't try and use them.

The following comment commands are no longer supported:

@dependabot merge
@dependabot cancel merge
@dependabot squash and merge
@dependabot close
@dependabot reopen