QUIC: removed use of SSL_quic_read_level and SSL_quic_write_level.

QUIC: removed use of SSL_quic_read_level and SSL_quic_write_level.

As explained in BoringSSL change[1], levels were introduced in the original
QUIC API to draw a line between when keys are released and when are active.
In the new QUIC API they are released in separate calls when it's needed.
BoringSSL has then a consideration to remove levels API, hence the change.

If not available e.g. from a QUIC packet header, levels can be taken based on
keys availability.  The only real use of levels is to prevent using app keys
before they are active in QuicTLS that provides the old BoringSSL QUIC API,
it is replaced with an equivalent check of c->ssl->handshaked.

This change also removes OpenSSL compat shims since they are no longer used.
The only exception left is caching write level from the keylog callback in
the internal field which is a handy equivalent of checking keys availability.

[1] https://boringssl.googlesource.com/boringssl/+/1e859054