Http2Adapter uses always http2 even when endpoint doesn't support it

[FrantisekGazo]

Issue Info

Info	Value
Platform Name	flutter
Platform Version	1.20.4
Dio Version	3.0.10
Http2Adapter Version	1.0.1
Android Studio / Xcode Version	Android Studio 4.0.1
Repro rate	all the time (100%)
Repro with our demo prj	no

Issue Description and Steps

When I make post request to an endpoint that doesn't support http2 the request fails with error like here #486 , but it doesn't happen if endpoint supports http2.

The http1 should still be supported as a fallback and not force to use http2 always.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If this is still an issue, please make sure it is up to date and if so, add a comment that this is still an issue to keep it open. Thank you for your contributions.

[sd148684]

We are facing the same issue here, is there any plan for implement a http/1.1 downgrade strategy, or have a workaround at least?