- Use CAGradientLayer for linear gradient (#52096)

Summary:
This PR replaces Core Graphics implementation with Core Animation for linear gradients. I came across a great [solution](https://stackoverflow.com/questions/38821631/cagradientlayer-diagonal-gradient/43176174#43176174) that makes the `CAGradientLayer`'s start and end point behaviour CSS spec compliant. This will make gradients much more performant.

## Changelog:

[IOS] [CHANGED] - Optimised Linear Gradients.
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Pull Request resolved: https://github.com/facebook/react-native/pull/52096

Test Plan:
Non breaking change. Test Linear gradient example from RNTester. Compare results with web, android and iOS. Each platform should render the gradients identically.

## Note:

I will be doing a PR to use `CAGradientLayer` for radial gradients as well. The next properties that I have locally working are `background-size`, `background-position` and `background-repeat`. These will be addressed in small PRs.

Reviewed By: NickGerleman

Differential Revision: D76905215

Pulled By: javache

fbshipit-source-id: 0094bdf70869d619272d491dd496983316b0dbf0