Drop Support for CloudEvents 0.3

[grant]

This PR drops support for 0.3 CloudEvents.
The main discussion of whether this should be done should continue in the issue (#99).

[lance]

So, this has been discussed ad nauseum in #99 and on Slack. I am opposed to this change until the 2.0 version of the CE specification has been released. I currently have code that receives Cloud Events from various, unknown sources - some of which are still sending using the 0.3 protocol. I do not want to have this code fail if these event producers are still producing 0.3 events. Until the 2.0 version is released (or release candidate) and there is an implementation of that protocol I think 0.3 should remain.

The SDK requirements doc says this about version compatibility.

• Each SDK has active points of contact.
• Each SDK supports the latest(N), and N-1, major releases of the CloudEvent spec*.
• Within the scope of a major release, only support for the latest minor version is needed.
• Support for release candidates is not required, but strongly encouraged.
• Note: v1.0 is a special case and it is recommended that as long as v1.0 is the latest version, SDKs should also support v0.3.

I don't really see the justification for completely dropping support and breaking things for existing users, contrary to recommendations. We will need to continue supporting multiple protocol versions down the line, so this to me seems short sighted.

[helio-frota]

SDKs should also support v0.3.

That seems to be the situation that needs justification according to the RFC which is followed by the spec

So, this has been discussed ad nauseum in #99 and on Slack. I am opposed to this change until the 2.0 version of the CE specification has been released. I currently have code that receives Cloud Events from various, unknown sources - some of which are still sending using the 0.3 protocol. I do not want to have this code fail if these event producers are still producing 0.3 events. Until the 2.0 version is released (or release candidate) and there is an implementation of that protocol I think 0.3 should remain.

I think that is a valid justification so 👎 for dropping 0.3 now.