Gemfile: Update Gitaly Gem to v14.8.0-rc1

We're about to make use of a new error type introduced for the
UserMergeBranch RPC, which requires us to upgrade to v14.8.0-rc1 of the
Gitaly Gem. Update our Gemfile to prepare for this.

Gemfile

@@ -473,7 +473,7 @@ gem 'ssh_data', '~> 1.2'
 gem 'spamcheck', '~> 0.1.0'
 
 # Gitaly GRPC protocol definitions
-gem 'gitaly', '~> 14.6.0.pre.rc1'
+gem 'gitaly', '~> 14.8.0.pre.rc1'
 
 # KAS GRPC protocol definitions
 gem 'kas-grpc', '~> 0.0.2'

Gemfile.lock

@@ -449,7 +449,7 @@ GEM
       rails (>= 3.2.0)
     git (1.7.0)
       rchardet (~> 1.8)
-    gitaly (14.6.0.pre.rc1)
+    gitaly (14.8.0.pre.rc1)
       grpc (~> 1.0)
     github-markup (1.7.0)
     gitlab (4.16.1)
@@ -1466,7 +1466,7 @@ DEPENDENCIES
   gettext (~> 3.3)
   gettext_i18n_rails (~> 1.8.0)
   gettext_i18n_rails_js (~> 1.3)
-  gitaly (~> 14.6.0.pre.rc1)
+  gitaly (~> 14.8.0.pre.rc1)
   github-markup (~> 1.7.0)
   gitlab-chronic (~> 0.10.5)
   gitlab-dangerfiles (~> 2.8.0)