Simplify DateUtils with ISO8601Utils

[denrase]

📜 Description

Replace usage of SimpleDateFormat with a helper class from GSON library.

Closes #1489

💡 Motivation and Context

It is faster and CG friendlier per their own documentation.

Closes #1489

💚 How did you test it?

Current unit tests for date helper class.

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

🔮 Next steps

[codecov-commenter]

Codecov Report

Merging #1837 (a350b1d) into 6.x.x (79945ce) will decrease coverage by 0.08%.
The diff coverage is 77.12%.

@@             Coverage Diff              @@
##              6.x.x    #1837      +/-   ##
============================================
- Coverage     80.93%   80.85%   -0.09%     
- Complexity     2850     2878      +28     
============================================
  Files           207      208       +1     
  Lines         10475    10613     +138     
  Branches       1380     1411      +31     
============================================
+ Hits           8478     8581     +103     
- Misses         1510     1525      +15     
- Partials        487      507      +20     

Impacted Files	Coverage Δ
...y/vendor/gson/internal/bind/util/ISO8601Utils.java	76.82% <76.82%> (ø)
sentry/src/main/java/io/sentry/DateUtils.java	100.00% <100.00%> (ø)

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[bruno-garcia]

Just missing a change log, otherwise,

[marandaneto]

I've fixed the javadocs issues and sharing the TIMEZONE_UTC field from the Util class

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- Simplify DateUtils with ISO8601Utils ([#1837](https://github.com/getsentry/sentry-java/pull/1837))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

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