[proposal] remove the `build` subject (for 0.6?)

[davidB]

following #109 and meeting of 2025-08-19

build could be removed for 0.5; it's a duplicate of pipelineRun or taskRun.

Migration path (from 0.4): It is recommended to convert build into taskRun and provide complementary information via links or custom data (to be defined).

I'll do the PR to remove the build as soon as possible before the release of 0.5 (after the merge of the pending PR that modifies build to avoid conflicts).

[xibz]

We need to properly define what all subject terms mean:

1. build - what is it
2. pipeline - what constitutes a pipeline
3. ...
   etc

As CDEvents, the original idea was to depend on tooling definitions of what a build is. However, that is hard to make interoperable. So instead we need CDEvent based definitions, and tooling can send an event that matches the criteria of whatever it is trying to describe.