security.rb: Reorder case branches in create_key to match help text

The following help text and error message in lib/rubygems/security.rb show that
the order of algorithms is RSA, DSA, EC. Align the case branches in create_key with
this order.

```
-A, --key-algorithm ALGORITHM    Select key algorithm for --build from RSA, DSA, or EC. Defaults to RSA.
```

```
"#{algorithm} algorithm not found. RSA, DSA, and EC algorithms are supported."
```

Assisted-by: Claude:Opus 4.6

Author: junaruga

lib/rubygems/security.rb

@@ -470,10 +470,10 @@ def self.create_digest(algorithm = DIGEST_NAME)
   def self.create_key(algorithm)
     if defined?(OpenSSL::PKey)
       case algorithm.downcase
-      when "dsa"
-        OpenSSL::PKey::DSA.new(RSA_DSA_KEY_LENGTH)
       when "rsa"
         OpenSSL::PKey::RSA.new(RSA_DSA_KEY_LENGTH)
+      when "dsa"
+        OpenSSL::PKey::DSA.new(RSA_DSA_KEY_LENGTH)
       when "ec"
         OpenSSL::PKey::EC.generate(EC_NAME)
       else