chore: set swift-tools-version to 6.2 (#16862)

The patch component in `swift-tools-version:6.2.1` has no effect on
the available PackageDescription APIs, which are versioned by
major.minor only. The actual minimum (Xcode 26.2 / Swift 6.2.3) is
enforced by the `#if compiler(<6.2.3)` check in Package.swift.

Using 6.2 also lets Swift 6.2.0 toolchains load the manifest and
reach that check's descriptive error, instead of failing early with a
generic tools-version mismatch.

FirebaseCore/CHANGELOG.md

@@ -30,10 +30,9 @@
   `GTMSessionFetcher` is now `4.0.0` (support for `3.x` has been dropped).
 - [changed] **Breaking change**: Bundle ID prefixes in the zip distribution are now
   properly scoped to `com.google.firebase`, instead of `org.cocoapods.`. (#16678)
-- [changed] Firebase now requires Swift tools version 6.2.1 and the Swift 6.2.3
-  compiler for the Swift Package. The package will no longer resolve in Xcode
-  versions older than 26.2, which remains the minimum officially supported
-  version for the SDK. (#16696)
+- [changed] Firebase now requires the Swift 6.2.3 compiler or newer. The SwiftPM
+  package will no longer resolve in Xcode versions older than 26.2, which
+  remains the minimum officially supported version for the SDK. (#16696)
 
 # Firebase 12.19.1
 - [fixed] Fixed the zip distribution artifact naming on the GitHub release

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:6.2.1
+// swift-tools-version:6.2
 // The swift-tools-version declares the minimum version of the Swift toolchain required to
 // build this package. Note: Firebase requires Xcode 26.2+, which includes the Swift 6.2.3 compiler.