Update RSA private key size to 2048 in test.

This is required to unblock updating our
`cryptography` pin:

aws#8895

This test was ported 5 years ago as is using a key
size of 512:

aws#4846

As of `cryptography==43.0.0` the minimum key size
allowed is 512:

pyca/cryptography@83dcbc1

This change bumps it up to a more reasonable value
of 2048. This has only a couple hundredths of a
second time difference to generate as tested on my
Mac.

tests/unit/customizations/cloudtrail/test_validation.py

@@ -362,7 +362,7 @@ def setUp(self):
         self._digest_data['_signature'] = 'aeff'
 
     def test_validates_digests(self):
-        private_key = rsa.generate_private_key(65537, 512, default_backend())
+        private_key = rsa.generate_private_key(65537, 2048, default_backend())
         sha256_hash = hashlib.sha256(self._inflated_digest)
         string_to_sign = "%s\n%s/%s\n%s\n%s" % (
             self._digest_data['digestEndTime'],