Restrict google-protobuf dependency to version < 3.15

Gitlab does not work with google-protobuff 3.15.x.
Using the new version causes segfaults:
/usr/local/lib/libruby27.so.27(rb_profile_frame_full_label+0x1665)
   [0x80059c535]
/usr/local/lib/libruby27.so.27(0x8003fa3fb) [0x8003fa3fb]
/usr/local/lib/libruby27.so.27(0x8005094d2) [0x8005094d2]
/lib/libthr.so.3(_pthread_sigmask+0x53e) [0x800649b7e]
/lib/libthr.so.3(0x80064913f) [0x80064913f]
   [0x7ffffffff193]
/usr/local/lib/ruby/gems/2.7/gems/google-protobuf-3.15.0/lib/google/
	   protobuf_c.so(Defs_register+0xc99) [0x811958699]
/usr/local/lib/ruby/gems/2.7/gems/google-protobuf-3.15.0/lib/google/
   protobuf_c.so(0x81195c675) [0x81195c675]

For more details see this bug report:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254010

Gemfile

@@ -476,7 +476,7 @@ gem 'gitaly', '~> 13.9.0.pre.rc1'
 
 gem 'grpc', '~> 1.30.2'
 
-gem 'google-protobuf', '~> 3.12'
+gem 'google-protobuf', '~> 3.14.0'
 
 gem 'toml-rb', '~> 1.0.0'
 

Gemfile.lock

@@ -493,7 +493,7 @@ GEM
       signet (~> 0.12)
     google-cloud-env (1.4.0)
       faraday (>= 0.17.3, < 2.0)
-    google-protobuf (3.12.4)
+    google-protobuf (3.14.0)
     googleapis-common-protos-types (1.0.5)
       google-protobuf (~> 3.11)
     googleauth (0.14.0)
@@ -1427,7 +1427,7 @@ DEPENDENCIES
   gitlab_omniauth-ldap (~> 2.1.1)
   gon (~> 6.2)
   google-api-client (~> 0.33)
-  google-protobuf (~> 3.12)
+  google-protobuf (~> 3.14.0)
   gpgme (~> 2.0.19)
   grape (~> 1.5.2)
   grape-entity (~> 0.7.1)