release: review test plan template

[endorama]

The test plan template include some sections which may not fall anymore into the test plan:

• go agent
• lambda extension
• k8s attacher

We should also add to the template the review of apm-data and go-docappender changes, specifying that in the test plan should only be included the single entries that brought functional changes (and not all changes introduced in the dependencies).

/cc @simitt