Change "Maintainer Response to Merge Request Duration" dashboard name

[mafesan]

My proposal is to update the name of "Maintainer Response to Merge Request Duration" dashboard under chaoss-gmd-cde section.

A possible new name could be "Maintainer Response to Pull Request Duration".

The rationale for this change is that this panel focuses on GitHub Pull Requests (it uses github_issues index), and with the current name this dashboard it could be wrongly interpreted as it was referring to GitLab Merge Requests instead of GitHub Pull Requests.

[alpgarcia]

The name should come from the metric implemented on the dashboard. This dashboard was created from a metric defined in CHAOSS Evolution Working Group. I wonder whether after the release of the CHAOSS metrics this dashboard should be updated or deprecated.