Use ec-256 as default key length

fix https://github.com/acmesh-official/acme.sh/issues/2350#issuecomment-1324029469

acme.sh

@@ -53,8 +53,8 @@ CA_SERVERS="$CA_ZEROSSL,$CA_LETSENCRYPT_V2,$CA_LETSENCRYPT_V2_TEST,$CA_BUYPASS,$
 
 DEFAULT_USER_AGENT="$PROJECT_NAME/$VER ($PROJECT)"
 
-DEFAULT_ACCOUNT_KEY_LENGTH=2048
-DEFAULT_DOMAIN_KEY_LENGTH=2048
+DEFAULT_ACCOUNT_KEY_LENGTH=ec-256
+DEFAULT_DOMAIN_KEY_LENGTH=ec-256
 
 DEFAULT_OPENSSL_BIN="openssl"