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

@@ -114,7 +114,7 @@
 
     context 'when passing a non existing username to the id' do
       it 'raises ActiveRecord::RecordNotFound' do
-        found = described_class.new("non_existent_username")
+        finder = described_class.new("non_existent_username")
 
         expect { finder.find_by_id! }.to raise_error(ActiveRecord::RecordNotFound)
       end
@@ -122,7 +122,7 @@
 
     context 'when passing an existing username to the id' do
       it 'raises ActiveRecord::RecordNotFound' do
-        found = described_class.new(user.username)
+        finder = described_class.new(user.username)
 
         expect { finder.find_by_id! }.to raise_error(ActiveRecord::RecordNotFound)
       end
@@ -130,7 +130,7 @@
 
     context 'when passing a non existing id to the username' do
       it 'raises ActiveRecord::RecordNotFound' do
-        found = described_class.new(1)
+        finder = described_class.new(1)
 
         expect { finder.find_by_username! }.to raise_error(ActiveRecord::RecordNotFound)
       end
@@ -138,7 +138,7 @@
 
     context 'when passing an existing id to the username' do
       it 'raises ActiveRecord::RecordNotFound' do
-        found = described_class.new(user.id)
+        finder = described_class.new(user.id)
 
         expect { finder.find_by_username! }.to raise_error(ActiveRecord::RecordNotFound)
       end
@@ -146,7 +146,7 @@
 
     context 'when passing an existing id (as string) to the username' do
       it 'raises ActiveRecord::RecordNotFound' do
-        found = described_class.new(user.id.to_s)
+        finder = described_class.new(user.id.to_s)
 
         expect { finder.find_by_username! }.to raise_error(ActiveRecord::RecordNotFound)
       end