Fix SSH Detect Host Keys not working

Due to a change in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24245, the Detect
Host Key feature in the SSH mirroring stopped working.
`SshHostKey#primary_key` was being used instead of the hard-coded
`:id`. However, `SshHostKey#find_by` was expecting the symbolized `:id`
rather than the string `id`, so it could never find the host key it was
supposed to update.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56855

app/models/ssh_host_key.rb

@@ -54,7 +54,7 @@ class SshHostKey
 
   # Needed for reactive caching
   def self.primary_key
-    'id'
+    :id
   end
 
   def id

changelogs/unreleased/sh-fix-detect-host-keys.yml

+---
+title: Fix Detect Host Keys not working
+merge_request: 24884
+author:
+type: fixed

spec/models/ssh_host_key_spec.rb

@@ -50,6 +50,12 @@ describe SshHostKey do
 
   subject(:ssh_host_key) { described_class.new(project: project, url: 'ssh://example.com:2222', compare_host_keys: compare_host_keys) }
 
+  describe '.primary_key' do
+    it 'returns a symbol' do
+      expect(described_class.primary_key).to eq(:id)
+    end
+  end
+
   describe '#fingerprints', :use_clean_rails_memory_store_caching do
     it 'returns an array of indexed fingerprints when the cache is filled' do
       stub_reactive_cache(ssh_host_key, known_hosts: known_hosts)