prepare 8.1.0 release

[LaunchDarklyReleaseBot]

[8.1.0] - 2023-12-19

Changed:

• Added tests to ensure the SDK works correctly with Xcode 15
• Removed tests for Xcode 12.

[louis-launchdarkly]

I will reopen this as this can go as 8.0.2 as there is no behavior change, mainly changing the things that we test.