Update c# language version to 9

[peppy]

No description provided.

[peppy]

I've applied remaining changes in line with what was done on the framework side, plus bumped the framework here (will require a fix, nuget not working for me right now so can't immediately address, feel free to fix anyone else).

[bdach]

Build failures cannot be easily resolved, due to the following TextureLoaderStore usage in example ruleset:

osu/Templates/Rulesets/ruleset-example/osu.Game.Rulesets.Pippidon/PippidonRuleset.cs

Lines 53 to 56 in 2ca4184

	public override Drawable CreateIcon() => new Sprite
	{
	Texture = new TextureStore(new TextureLoaderStore(CreateResourceStore()), false).Get("Textures/coin"),
	};

The ctor in question was internalised framework-side in ppy/osu-framework#5240 and now there is no good way to access GameHost from the ruleset to create a texture loader store.

Related: #11240

May want to revert that particular breaking change framework-side for now.

[peppy]

Painful. Wanna make a PR reverting that if you're able to?

[bdach]

Done (ppy/osu-framework#5266).

[bdach]

Bumped framework again (manually, so buyer beware), as well as fixed the failures from the framework breaking change to texture stores in a83c45b. @frenzibyte would appreciate a double-check of the latter if/when able, if it matches the changes you had queued.

[bdach]

How were these dotsettings changes made? Looks like the file may have been copy-pasted verbatim from the framework directory to the game's, but in the meantime this subtly changed the set of enabled inspections (like the "async void" one above).

[peppy]

Overwrote the file with framework version and undid anything that looked out of place (or caused changes in warnings/errors).

Welcome to remove these changes if you want to look into it further, I was just making sure it didn't affect already existing code, mostly.

[bdach]

I have changes queued that revert all inspection downgrades that happened as a result of the procedure you described, just want to run them past inspectcode again to make sure I haven't missed anything.

[bdach]

Pushed changes in e0c2228. Now the diff of osu.sln.DotSettings only contains additions of new rules (and one whitespace fix).