Deprecate --allow-warnings; Add --deny-warnings

[Savio-Sou]

PR: noir-lang/noir#1383

Warnings used to be treated as errors by default, where devs can opt out of the behavior with the --allow-warnings flag.

From v0.7.0 onwards, warnings are treated as warnings by default and the --allow-warnings flag is removed.

A --deny-warnings flag is added for opting in for treating warnings as errors.

[Savio-Sou]

@TomAFrench not exactly sure what you meant with noir-lang/noir#1383 (comment).

Would you like to elaborate on it a bit?

[TomAFrench]

Just that if we have any example CI workflows shown in the docs for people to copy-paste, then we should update the flags in there to disallow warnings.