fix incorrect FeatureNew for separator kwarg in environment object methods

In commit 6acfe48f32, the kwarg was added
to environment() in addition to the env object methods. As part of the
associated refactor, a shared KwargInfo was used, and evolved to be new
since 0.62.0 in the two cases where it is in fact new.

However, it *also* set the base KwargInfo for the exact same newness,
which is wrong as it was present ever since the initial introduction in
0.34.0

As usual for anything that predates 0.37.0 we simply don't tag
FeatureNew. Revert this back to the same KwargInfo definition from
before the refactoring commit.

Fixes #10402

mesonbuild/interpreter/type_checking.py

@@ -336,4 +336,4 @@ DEFAULT_OPTIONS: KwargInfo[T.List[str]] = KwargInfo(
 ENV_METHOD_KW = KwargInfo('method', str, default='set', since='0.62.0',
     validator=in_set_validator({'set', 'prepend', 'append'}))
 
-ENV_SEPARATOR_KW = KwargInfo('separator', str, default=os.pathsep, since='0.62.0')
+ENV_SEPARATOR_KW = KwargInfo('separator', str, default=os.pathsep)