Standardize C++ version naming

[Rageking8]

The canonical way to represent a C++ version (only the last 2 digits of the year) is without the extra space character (i.e. either C++20 or C++ 2020). This PR removes all extra spaces as the canonical version is more prevalent in this docs repository and is used as such in the official ISO website.

[Rageking8]

This is missing the C++20 label, added it as a drive-by.

[learn-build-service-prod]

Learn Build status updates of commit cbaebf8:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/build/walkthrough-header-units.md	✅Succeeded
docs/cpp/align-cpp.md	✅Succeeded
docs/cpp/compiler-limits.md	✅Succeeded
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docs/standard-library/iota-view-class.md	✅Succeeded

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[Jak-MS]

@TylerMSFT - is this yours to review? If not, you can reassign or let me know who to assign. Thanks!

• IMPORTANT: When this content is ready to merge, you must add #sign-off in a comment or the approval may get overlooked.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[TylerMSFT]

Nice cleanup - thanks @Rageking8

[TylerMSFT]

#sign-off