Updated the DateTime scalar to enforce a specific format

[glen-84]

Summary of the changes (Less than 80 chars)

• Updated the DateTime scalar to enforce a specific format.

Closes #6276

---

Differences to DateTime Scalar spec:

• Fractions of a second are optional, and 0-7 digits may be specified.

Differences to RFC 3339:

• A negative zero offset (-00:00) is disallowed.
  • https://www.graphql-scalars.com/date-time/#no-unknown-local-offset-convention
• A maximum of 7 digits may be specified as fractions of a second.

⚠️ Breaking change.

[codecov]

Codecov Report

Attention: Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.09%. Comparing base (0b209c3) to head (e606016).

Files	Patch %	Lines
...olate/Core/src/Types/Types/Scalars/DateTimeType.cs	82.35%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7329      +/-   ##
==========================================
+ Coverage   77.04%   77.09%   +0.04%     
==========================================
  Files        2459     2781     +322     
  Lines      121972   138774   +16802     
==========================================
+ Hits        93968   106981   +13013     
- Misses      28004    31793    +3789     

Flag	Coverage Δ
unittests	77.09% <82.35%> (+0.04%)	⬆️

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