Symfony version(s) affected: 4.1

Description
Currently it's allowed to do define a scalar value for a default env parameter, e.g. env(SOME): 1.5 (opposed to its string value"1.5") while that's not possible with real env vars (hence we added the prefix processors). It's a trap and as such I think we should deprecate non-string defaults, and force string values in 5.0.

This way we can safely imply a default string:... processor again, and cast to i.e. float using its dedicated float:... processor. It was reverted in #27470 to preserve BC (see #27455), but i tend to agree with @stof in #27470 (comment) this is not correct.

cc @nicolas-grekas

edit: this also requires to forbid setting prefixed defaults, which is possible today but not handled as such. I'd keep this possibility closed until then... today those are silently ignored.