Add support for Flask >= 3.0.0

[radohristov]

Before opening a feature request against this repo, consider whether the feature should/could be implemented in the other OpenCensus client libraries. If so, please open an issue on opencensus-specs first.

Is your feature request related to a problem?
The Opencensus extension for Flask is not compatible with the most recent versions of the Flask:
opencensus-ext-flask 0.8.2 requires flask!=1.1.3,<3.0.0,>=0.12.3, but you have flask 3.0.0 which is incompatible.

Describe the solution you'd like.
Please, add support for the Flask ( >= 3.0.0)

Describe alternatives you've considered.
None

Additional context.
Our project requires an upgrade to the latest Flask but this Opencensus extension for Flask is blocking it.