Rename `pipeline` to `tasks`. (#8157)
### Description
Renaming the `pipeline` key to `tasks` in turbo.json.
We went this key to better express what Turborepo does with the
configuration that is provided in this key. We've seen with users that
the term "pipeline" can be confusing for how `turbo` _really_ executes
the task graph.
In reality, the keys in the `pipeline` object are really the list of
_tasks_ registered to `turbo` - so let's call it that.
### Non-goals
This PR does not include update:
- The examples in the repo
- JS packages
- `eslint-plugin-turbo`
These updates will be needed in subsequent PRs.
### Testing Instructions
Hopefully CI will do its job here - but, of course, would appreciate a
review of my changes to make sure I'm not missing anything my untrained
eye may be missing.
CLOSES TURBO-3225
