Remove TP notice from the documentation for the Camel-based policy extension mechanism

After a round of discussion between the 3scale QE and Fuse QE teams, the conclusion was reached that we fully support the Camel-based policy extension mechanism in 3scale after all. So, we should remove any and all Technology Preview admonitions from the existing documentation.

[1] https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.7/html/administering_the_api_gateway/transform-with-policy-extension

Note: Reminder to check back with Jakub Smolar in QE to see if any other updates are necessary (e.g., for Camel 2.0/3.0 code sample)