Make getting a user by the username case insensitive

What does this MR do?

This merge request makes sure that when creating a new user object searching by username, it does so in a case sensitive manor.

What are the relevant issue numbers?

Closes #38304 (closed)
Closes #27229 (closed)
Possibly solves #47601 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated (After searching, nothing in the docs suggest usernames are sensitive or insensitive),
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

spec/finders/user_finder_spec.rb

@@ -6,7+6,7 @@
   describe '#execute' do
     context 'when the user exists (id)' do
       it 'returns the user' do
         user = create(:user)
         found = described_class.new(user.id).execute
 
         expect(found).to eq(user)
       end
     end
 
+    context 'when the user exists (id as string)' do
+      it 'returns the user' do
+        user = create(:user)
+        found = described_class.new(user.id.to_s).execute
+
+        expect(found).to eq(user)
+      end
+    end
+
     context 'when the user exists (username)' do
       it 'returns the user' do
         user = create(:user)
@@ -22,6 +31,14 @@
       end
     end
 
+    context 'when the user does not exist (username)' do
+      it 'returns nil' do
+        found = described_class.new("non_existent_username").execute
+
+        expect(found).to be_nil
+      end
+    end
+
     context 'when the user does not exist' do
       it 'returns nil' do
         found = described_class.new(1).execute
@@ -41,6 +58,15 @@
       end
     end
 
+    context 'when the user exists (id as string)' do
+      it 'returns the user' do
+        user = create(:user)
+        found = described_class.new(user.id.to_s).execute!
+
+        expect(found).to eq(user)
+      end
+    end
+
     context 'when the user exists (username)' do
       it 'returns the user' do
         user = create(:user)
@@ -50,6 +76,14 @@
       end
     end
 
+    context 'when the user does not exist (username)' do
+      it 'raises ActiveRecord::RecordNotFound' do
+        finder = described_class.new("non_existent_username")
+
+        expect { finder.execute! }.to raise_error(ActiveRecord::RecordNotFound)
+      end
+    end
+
     context 'when the user does not exist' do
       it 'raises ActiveRecord::RecordNotFound' do
         finder = described_class.new(1)