Remove REQUIRED, main branch (2023.10.12.)

Removed the REQUIRED keyword from all find_package(...) calls that look for "externals".

This is meant to make our builds more robust against build errors. For instance in the recent dev3/dev4 nightlies ROOT was missing. Which stopped the builds completely. In fact, it is still missing in this nightly:

https://bigpanda.cern.ch/nview/?nightly=main--dev4LCG_Athena_aarch64-el9-gcc11-opt

And while it's true that without a major external like ROOT not a whole lot from our projects will build successfully, the build should at least be attempted even in that case.