[Swift] Increase platform support to iOS 12 to avoid deprecated warning

[vsarunas]

Compiling with Swift 6.0 and newer creates a build warning:

flatbuffers/Package.swift:23:11: warning: 'v11' is deprecated: iOS 12.0 is the oldest supported version
21 |   name: "FlatBuffers",
22 |   platforms: [
23 |     .iOS(.v11),
   |           `- warning: 'v11' is deprecated: iOS 12.0 is the oldest supported version
24 |     .macOS(.v10_14),
25 |   ],

iOS was released in 2017.

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[mustiikhalil]

Thanks for opening the PR,

This was part of the following PR #8577, so I will close it