CI: switch ios-armv7 runner to XCode 15

In my tests 15.4 is the last XCode that can build armv7. 16.4 and above fail:

ld: warning: ignoring file /Volumes/APFS/opt/Xcode16.4.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS18.5.sdk/usr/lib/libSystem.tbd, missing required architecture armv7 in file /Volumes/APFS/opt/Xcode16.4.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS18.5.sdk/usr/lib/libSystem.tbd (2 slices)
ld: dynamic executables or dylibs must link with libSystem.dylib for architecture armv7
clang: error: linker command failed with exit code 1 (use -v to see invocation)

As long as we support armv7 we cannot go above this one. But we can use the latest one possible to allow building long in the future.