Currently only HTTP/1.1 is supported by the RFC6455 to be used for connection upgrade.
Once a connection to the server has been established (including a
connection via a proxy or over a TLS-encrypted tunnel), the client
MUST send an opening handshake to the server. The handshake consists
of an HTTP Upgrade request, along with a list of required and
optional header fields. The requirements for this handshake are as
follows.
The handshake MUST be a valid HTTP request as specified by
[RFC2616].
The method of the request MUST be GET, and the HTTP version MUST
be at least 1.1.
However, there are proposals to support HTTP/2 as well as HTTP/3 (as Remy Lebeau mentioned in comment above).