core/sr-api-macros/Cargo.toml: Pin protobuf version (#3723)

* Cargo.toml: Pin protobuf version

The protobuf crate introduced a breaking change within its semver minor
update from 2.8.1 to 2.9.0. This commit ensures Substrate uses anything
within the 2.8 minor releases.

* Cargo.lock: Update

* core/sr-api-macros/Cargo.toml: Pin protobuf version

The protobuf crate introduced a breaking change within its semver minor
update from 2.8.1 to 2.9.0. This commit ensures Substrate uses anything
within the 2.8 minor releases.

* Revert "Cargo.toml: Pin protobuf version"

This reverts commit 4e293c74.

* core/sr-api-macros/Cargo.toml: Pin protobuf within dev-dependencies

* core/sr-api-macros/Cargo.toml: Add comment and link to Githu issue