Add support for react native 0.68

[AlexanderEggers]

Is your feature request related to a problem? Please describe.
React native published the stable version of 0.68. I noticed that this library excludes support for that version - but there is actually no reason for that. Of course the new architecture won't work, but at least the old one should still work without issues.

Describe the solution you'd like
Update the peerDependencies (and any other needed changes) to support react native 0.68 officially.

[louis-launchdarkly]

Hello @AlexanderEggers, thank you for reaching out to us and letting us know RN 0.68 now has a stable version. Because we will need to update the test to run against the new environment, we can't immediately pick up your PR. I will provide another update when we finish testing the change.

[louis-launchdarkly]

As a quick update, from the testing on our private mirror, it seems RN 0.68 is working fine with the current RN SDK code. We are going to cut a release for that, and I will give you another update when LaunchDarkly's official RN 0.68 support is released.

[louis-launchdarkly]

Added the support for RN 0.68 in 6.1.3.