Generic 'index_builder' is Mercurial specific

[ahal]

The generic 'index_builder' that gets used by default adds routes referencing pushlog-id and such:
https://github.com/taskcluster/taskgraph/blob/main/src/taskgraph/transforms/task.py#L180

We should try and make it truly generic across VCS types as well. We'll either need to move the functionality the current one provides to a new Mercurial specific builder, or perhaps we can just get Gecko to define it if it's the only consumer that needs those routes.

[ahal]

It might be possible to implement this in a way that is backwards compatible, but probably easier to just do it across a major version bump.

[gabrielbusta]

Updated link: https://github.com/taskcluster/taskgraph/blob/6f37d7805c9897b6b044051ac0dcf66dde5b0e76/src/taskgraph/transforms/task.py#L205C40-L205C50