[confmap] Restrict characters in environment variable URIs even more!

[mx-psi]

In #5706, we discussed restricting URIs in characters for all providers. We ended up disallowing $ in #6268 for the recursive implementation done later.

I think we should restrict the character set even more for the env provider, in particular by following the restrictions defined by the OpenTelemetry Configuration Working Group (see here). While, as mentioned in #5706 (comment), there are more characters allowed on both Windows and Linux, the set of environment variable names allowed by the spec is reasonable and (pressumably!) what the vast majority of users use.

We should probably restrict it also on the expandconverter while it exists