Update minimum watchOS deployment target from deprecated v4 to watchOS 9

[zunda-pixel]

Summary

Xcode reports the following warning for the package manifest:

'v4' is deprecated: watchOS 9.0 is the oldest supported version

The warning comes from Package.swift, where the package still declares watchOS 4 support:

.watchOS(.v4)

Expected change

Update the Swift Package manifest to use the current oldest supported watchOS deployment target, likely:

.watchOS(.v9)

Also update documentation that still lists watchOS 4 support, including the README platform support section:

watchOS 4 or later.

Acceptance criteria

• Package.swift no longer uses deprecated .watchOS(.v4).
• README platform requirements match the package manifest.
• The package manifest no longer emits the watchOS v4 deprecation warning in Xcode.

Relevant locations

• Valet/Package.swift:11
• Valet/README.md:215