Standardize Canceled vs Cancelled in codebase

The following discussion from !11449 should be addressed:

• @cwiesner started a discussion: (+2 comments)

  thought (non-blocking): It looks like there's a mixture of the usages for cancelled and canceled. There are more hits in the code (without .xml and .yml files in CDot) for cancelled than canceled. This makes me wonder if this should be aligned to use the same style everywhere. WDYT, worth the (separate) effort?