Remove 'gray-matter' dependency because of security concerns

[6543]

currently the dependency maintainer is gone fro some time ...

-> https://github.com/jonschlinkert/gray-matter

and even simple version bumps are stale jonschlinkert/gray-matter#172

also it has security issues open for a long time: jonschlinkert/gray-matter#182

so this looks unmaintained and a security risk.

I propose to get rid of this dependency and implement it ourselves or use an alternative ...

[slorber]

Already handled, see #12174

[6543]

sorry just got notified of it - thanks for the work!!!