[gnutls-devel] GnuTLS | DTLS priority enables TLS1.2 (#1054)

[Development of GNU's TLS library]

Miroslav Lichvar created an issue: https://gitlab.com/gnutls/gnutls/-/issues/1054



## Description of problem:
On a TLS server, I'd like to disable all TLS versions before TLSv1.3. When I set the priority to
```
NORMAL:-VERS-SSL3.0:-VERS-TLS1.2:-VERS-TLS1.0:-VERS-TLS1.1
```
it seems clients can still connect using TLS1.2. But when I add there `-VERS-DTLS-ALL`, it works as expected. If DTLS1.0 or DTLS1.2 is enabled, the server allows the client to use TLS1.2. I think that's unexpected, or at least I couldn't find an explanation in the documentation.

## Version of gnutls used:
gnutls-3.6.14

## Distributor of gnutls (e.g., Ubuntu, Fedora, RHEL)
Fedora

## How reproducible:
Always

Steps to Reproduce:

 * gnutls-serv --x509keyfile=server.key --x509certfile=server.crt --priority="NORMAL:-VERS-TLS1.2:-VERS-TLS1.0:-VERS-TLS1.1"
 * gnutls-cli server.example.net -p 5556 --priority="NORMAL:-VERS-ALL:+VERS-TLS1.2"

## Actual results:
Handshake completed

## Expected results:
Handshake failed

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