Support latest Flagsmith SDKs

[danjuv]

Installing the provider via the documented guidance fails because the latest Flagsmith SDK is v5.1.1, and this provider requires <v4.0.0.

I'm unsure if there's any legacy reasons that require this version constraint, but it would be great to either:

1. Support latest Flagsmith SDKs, unsure if this is just moving the versioning constraints or there's legacy reasons that necessitate this only supports the v3 clients.
2. Document clearly (i.e. in the README) that this provider only supports Flagsmith@v3

If the fix is simple (just a README update or removing the versioning constraint), happy to help out as needed.

[danjuv]

cc @matthewelwell @khvn26 interested in your opinions on this