Azure Functions HTTP: use utf-8 instead of default charset decoding request

[Nithanim-Work]

This change removes the implicit usage of the default charset when decoding the request from the incoming Azure Functions invocation. It now always decodes the payload as UTF-8, which should match the grpc passing which is used internally in Azure.
Fix #36973

[quarkus-bot]

Thanks for your pull request!

The title of your pull request does not follow our editorial rules. Could you have a look?

• title should not contain an issue number (use Fix #1234 in the description instead)

_{This message is automatically generated by a bot.}

[quarkus-bot]

Status for workflow Quarkus CI

This is the status report for running Quarkus CI on commit bfb73c7.

✅ The latest workflow run for the pull request has completed successfully.

It should be safe to merge provided you have a look at the other checks in the summary.

You can consult the Develocity build scans.

[gsmet]

Thanks for the fix!

[Nithanim-Work]

@gsmet Thank you for your hard work!