Update google-protobuf to 3.15.8 to support Ruby 3.0

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

Gemfile

@@ -483,7 +483,7 @@ gem 'gitaly', '~> 13.11.0.pre.rc1'
 
 gem 'grpc', '~> 1.30.2'
 
-gem 'google-protobuf', '~> 3.14.0'
+gem 'google-protobuf', '~> 3.15.8'
 
 gem 'toml-rb', '~> 1.0.0'
 

Gemfile.lock

@@ -513,7 +513,7 @@ GEM
       signet (~> 0.12)
     google-cloud-env (1.4.0)
       faraday (>= 0.17.3, < 2.0)
-    google-protobuf (3.14.0)
+    google-protobuf (3.15.8)
     googleapis-common-protos-types (1.0.5)
       google-protobuf (~> 3.11)
     googleauth (0.14.0)
@@ -1463,7 +1463,7 @@ DEPENDENCIES
   gitlab_omniauth-ldap (~> 2.1.1)
   gon (~> 6.4.0)
   google-api-client (~> 0.33)
-  google-protobuf (~> 3.14.0)
+  google-protobuf (~> 3.15.8)
   gpgme (~> 2.0.19)
   grape (~> 1.5.2)
   grape-entity (~> 0.7.1)