Search repo for files with string replacement patterns rather than hard coding a list

[Ian Pittwood]

It would be useful to grep for version replacement patterns, such as ^R_VERSION := or ^R_VERSION=, to get the list of files to update instead of having to manually update a hard coded list that could be prone to human error.

We could even write a just command that would find the files matching a specific pattern and then send them as arguments to the sed command.

Originally proposed by @bschwedler in #588 (comment)

[Benjamin R. J. Schwedler]

Reported in #613, we will also need to add PYTHON_VERSION_JUPYTER to the list of versions to update, defaulting to the value of PYTHON_VERSION_ALT.