[deps] Update dependency protobuf to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
protobuf (source)	dependencies	major	5.1.0 → 6.0.0

---

Release Notes

google/protobuf.dart (protobuf)

v6.0.0

Compare Source

• New GeneratedMessage extension methods toTextFormat and writeTextFormat
  added to convert the message into the official protocol buffers text
  format. (#​1080, #​125)

• Add well-known proto types as libraries. This change is required for
  protoc_plugin-25.0.0. (#​1081)

• Breaking: Hide PbList and PbMap constructors. It is not possible to
  construct these values correctly in user code, so the constructors are now
  private. Existing uses of PbList can be replaced by List and PbMap can
  be replaced by Map.

  For immutable lists and maps, you can use built_value. (#​1072)

• Map fields now check key and value validity when adding elements. (#​1065,
  #​1076)

---

Configuration

📅 Schedule: Branch creation - "every 2 weeks on Monday" (UTC), Automerge - At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about this update again.

---

• If you want to rebase/retry this PR, check this box

---

This PR was generated by Mend Renovate. View the repository job log.

[renovate]

⚠️ Artifact update problem

Renovate failed to update an artifact related to this branch. You probably do not want to merge this PR as-is.

♻ Renovate will retry this branch, including artifacts, only when one of the following happens:

• any of the package files in this branch needs updating, or
• the branch becomes conflicted, or
• you click the rebase/retry checkbox if found above, or
• you rename this PR's title to start with "rebase!" to trigger it manually

The artifact failure details are included below:

File name: src/client/gui/pubspec.lock

Command failed: flutter pub upgrade protobuf
Because grpc 5.0.0 depends on protobuf ^5.1.0 and no versions of grpc match >5.0.0 <6.0.0, grpc ^5.0.0 requires protobuf ^5.1.0.
So, because multipass_gui depends on both grpc ^5.0.0 and protobuf 6.0.0, version solving failed.


You can try the following suggestion to make the pubspec resolve:
* Consider downgrading your constraint on protobuf: flutter pub add protobuf:^5.1.0
Failed to update packages.

[ricab]

This doesn't work standalone. We'd need to update it together with gRPC, probably other things too. Might be better to upgrade all flutter dependencies in one go, similar to #3655