feat: default to latest protobuf rather than pinning

[alexeagle]

bzlmod users got v25.3 previously, which was just arbitrary at a point-in-time and is now stale.

Update the install snippet to show users how to pick a version of protoc in case they don't want to pick up new protoc when they upgrade toolchains_protoc

Also fix that rules_go was accidentally a non-dev-dep

FYI @zhangskz, @mkruskal-google, @googleberg

---

Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): yes/no
• Suggested release notes appear below: yes

Bzlmod users now get the latest protoc by default. Pin to an older version if desired.

Test plan

• Covered by existing test cases

[thesayyn]

Makes sense to me.