renovatebot: group golang updates

What does this MR do and why?

With the current configuration golang updates are split into two merge requests:

• !1874 (closed) - .tools-version
• !1861 (closed) - dockerfiles

With this change we tell renovatebot to bundle them into an atomic upgrade like the following one on my fork nolith/release-tools!7 (closed)

Author Check-list

• Has documentation been updated?