Make Zeek 5.1 compatible

[awelzel]

Currently this plugin fails to compile with Zeek 5.1 due to the following error:

src/HTTP2.cc:146:13: error: ‘ProtocolConfirmation’ was not declared in this scope; did you mean ‘LN_protocolInformation’?
  146 |             ProtocolConfirmation(); // Notify system that this is HTTP2.
      |             ^~~~~~~~~~~~~~~~~~~~
      |             LN_protocolInformation
src/HTTP2.cc:158:17: error: ‘ProtocolViolation’ was not declared in this scope
  158 |                 ProtocolViolation("Unable to parse http 2 frame from data stream, fatal error");

With Zeek 5.1 ProtocolConfirmation() and ProtocolViolation() have been removed. Since Zeek 4.2 there's AnalyzerConfirmation() and AnalyzerViolation() available.

[malwarefrank]

Thank you!