[TEP-0044] Clarify problem statement

This commit adds more detail about the friction associated with
scheduling every TaskRun in its own pod.

It also clarifies the design requirements, including removing the
requirement that Pipeline execution can be specified at runtime,
and restating a unclear requirement about replacing some features
of PipelineResources.

Lastly, it removes modified PipelineResources as a potential solution,
as they have been deprecated.