[InputLabel] Change size prop option from normal to medium

[TurtIeSocks]

• Replaces normal with medium
• More in line with size props in the rest of the package(s)
• As far as I can tell, this is the only component that uses small/normal as the size options, opposed to small/medium

I didn't leave it "backwards compatible" since the size prop is not used when it's set to default/normal anyway, but I can adjust that if normal should be type supported until the next major version.

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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[michaldudak]

Thanks for your PR. Indeed this seems like the only place we use the term "normal" when describing size.
However, even though this is the default value, there may be implementations where developers use it explicitly. We don't want to introduce breaking changes outside of major versions.
I'm happy to assign it to the v6 milestone and include it in that release.

[TurtIeSocks]

Sounds good to me.