Reduce allocations in BelongsToAssociation#replace_keys

Using the same benchmark as in #51726

`replace_keys` always allocate a multiple arrays to support composite
primary keys, even though most primary keys likely aren't composite.

And even when they are, we can avoid needless allocations by not using
`Array#zip`.

This reduce allocations by 18% (8k) on that benchmark.

Before:

```
Total allocated: 4.88 MB (44495 objects)
Total retained: 4.16 MB (32043 objects)

allocated memory by file
-----------------------------------
...
 320.00 kB  activerecord/lib/active_record/associations/belongs_to_association.rb

allocated objects by file
-----------------------------------
...
      8000  activerecord/lib/active_record/associations/belongs_to_association.rb
```

After:

```
Total allocated: 4.56 MB (36495 objects)
Total retained: 4.16 MB (32041 objects)
```

NB: `belongs_to_association` doesn't show up in top files anymore

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -124,12 +124,21 @@ def require_counter_update?
         end
 
         def replace_keys(record, force: false)
-          target_key_values = record ? Array(primary_key(record.class)).map { |key| record._read_attribute(key) } : []
-          reflection_fk = Array(reflection.foreign_key)
+          reflection_fk = reflection.foreign_key
+          if reflection_fk.is_a?(Array)
+            target_key_values = record ? Array(primary_key(record.class)).map { |key| record._read_attribute(key) } : []
+            reflection_fk = Array(reflection.foreign_key)
+
+            if force || reflection_fk.map { |fk| owner._read_attribute(fk) } != target_key_values
+              reflection_fk.each_with_index do |key, index|
+                owner[key] = target_key_values[index]
+              end
+            end
+          else
+            target_key_value = record ? record._read_attribute(primary_key(record.class)) : nil
 
-          if force || reflection_fk.map { |fk| owner._read_attribute(fk) } != target_key_values
-            reflection_fk.zip(target_key_values).each do |key, value|
-              owner[key] = value
+            if force || owner._read_attribute(reflection_fk) != target_key_value
+              owner[reflection_fk] = target_key_value
             end
           end
         end