Support variables in the project property of triggers

What does this MR do and why?

Resolves issue #367660 (closed), i.e. it adds support for variable expansion in the project property of the trigger keyword.

This is done in basically the same way as in !17430 (merged) for issue #10126 (closed) which did the same for the branch property of the trigger keyword, meaning that I added the expansion with a test and a line in the documentation.

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