[path_provider] Add missing pluginClass: none to path_provider_windows

[stuartmorgan-g]

Description

Older versions of Flutter (still supported by path_provider) need a
'pluginClass: none' to avoid "Invalid plugin specification" errors
when parsing a Dart-only desktop plugin like this one.

Related Issues

Fixes flutter/flutter#66158

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[stuartmorgan-g]

Adding a bunch of reviewers so whoever can get to it first can approve.

I don't understand how CI on the endorsement PR passed if stable still requires the pluginClass: none workaround.

[stuartmorgan-g]

Ah, I think this only happens for people who haven't updated to the current stable. 1.20 shouldn't need this. However, path_provider doesn't require 1.20+ currently, so this should still be added (so that the last version that claims to support older versions isn't broken on those versions).

[cyanglaz]

I think we can just make path_provider to require 1.20+ .

[stuartmorgan-g]

I think we can just make path_provider to require 1.20+ .

We could do that later, but if we don't push this first then the last published path_provider that "supports" 1.17 will be broken for 1.17, which is not a good situation to permanently leave the ecosystem in.