Disable ExistentialAny temporarily (#9)

Disable ExistentialAny temporarily

### Motivation

Unfortunately until we adopt 5.9, adding ExistentialAny on upstream packages has unintended consequences for some downstream packages, so disabling for now. Details in apple/swift-openapi-generator#120

### Modifications

Disabled the feature enforcement, but the code changes are there, so downstream adopters can still use them.

### Result

We won't be seeing the issue described in apple/swift-openapi-generator#120.

### Test Plan

PR CI, which discovered the original issue.


Reviewed by: simonjbeaumont

Builds:
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#9

Relax deployment target to macOS 10.15 (#6)

### Motivation

Relax deployment target to macOS 10.15

### Modifications

- Reduced requirement from macOS 13 to macOS 10.15
- Added the minimum version to readme and documentation

### Result

- Users will be able to deploy it on projects with a minimum deployment
target of 10.15

Initial commit

Co-authored-by: Honza Dvorsky <[email protected]>
Co-authored-by: Si Beaumont <[email protected]>
Co-authored-by: David Nadoba <[email protected]>