[Firebase AI] Update podspecs to Swift 6.0

[andrewheard]

Updated FirebaseAI.podspec and FirebaseAILogic.podspec to swift_version = '6.0' to support Swift Testing unit tests and and to match Package.swift. This currently overwritten to 6.0 in our pod-lib-lint jobs, except on macos-14:

firebase-ios-sdk/.github/workflows/common_cocoapods.yml

Lines 143 to 145 in bf942b9

	- name: Set podspec Swift version to 6.0, if supported.
	if: inputs.supports_swift6 == true && matrix.os != 'macos-14'
	run: sed -i "" "s/s.swift_version[[:space:]]*=[[:space:]]*'5.9'/s.swift_version = '6.0'/" ${{ inputs.product }}.podspec

We only support Xcode 16+ now which includes the Swift 6.0+ toolchain, even on macOS 14.

#no-changelog

[andrewheard]

cc: @ncooke3 (no need to look at this over the long weekend, I'll double check that there's no impact on the nightly zip build tomorrow)