[TC] [HTTP] [UTIL] Retry HTTP2 force closure errors

What's the issue?
The 'http2: client connection force closed via ClientConn.Close' error can be returned when the remote server force closes the connection.

Assuming the remote server is written in Go - which may be sensible given it's a Google service - this error can be returned after a panic (source) where the server aborts the request being served.

What's the fix?
We should extend add this error to our 'TemporaryErrorMessages' type so that this error is retried in tools such as cbbackupmgr.