fix leak in strip digi, and switch it on for ITk by default

Following up from !41344 (merged) this fixes a leak that was found during testing, and make the new StripDigitization package run by default for ITk. This is initially with a view to testing it more extensively, which was agreed with @tstreble and @ncalace, since some small differences in the results are seen. This appear to be fluctuations, perhaps related to different random numbers being generated/picked up, rather than a genuine functional difference or bug, but larger scale tests are required to see that for sure (hence the motivation for merging).

A small test showing the nature of the effect on RDOs can be seen here:

http://nstyles.web.cern.ch/nstyles/dcube/digi/

Digest reference is updated. If there are still features to be understood in larger-scale testing, this MR can be reverted until they are understood.