Add --skip-validation option synonym for --skip-valid

[evgenyz]

First step on the way to deprecate and then remove misleading option --skip-valid (why on earth should we skip something that is valid?).

[jan-cerny]

@evgenyz What about deprecating it right now? I mean keep --skip-valid working, but document it as deprecated and prefer --skip-validation everywhere.

[evgenyz]

@evgenyz What about deprecating it right now? I mean keep --skip-valid working, but document it as deprecated and prefer --skip-validation everywhere.

I'm not against deprecating it in the documentation (I kind of already moved the accent away from it in this PR), but emission of a warning could break some tests. Also I changed all --skip-valid invocations in the project to --skip-validation.

[jan-cerny]

It is a known issue: actions/runner-images#2703 and they say they will roll out new image soon.

[evgenyz]

It is a known issue: actions/virtual-environments#2703 and they say they will roll out new image soon.

QA? No, never heard about it.