crypto: fix/update borg version comments

new AEAD crypto can be used with borg >= 1.3.
old crypto is used by attic and borg < 1.3.

src/borg/crypto/low_level.pyx

@@ -184,7 +184,7 @@ class UNENCRYPTED:
 
 
 cdef class AES256_CTR_BASE:
-    # Layout: HEADER + MAC 32 + IV 8 + CT (same as attic / borg < 1.2 IF HEADER = TYPE_BYTE, no AAD)
+    # Layout: HEADER + MAC 32 + IV 8 + CT (same as attic / borg < 1.3 IF HEADER = TYPE_BYTE, no AAD)
 
     cdef EVP_CIPHER_CTX *ctx
     cdef unsigned char *enc_key
@@ -423,6 +423,7 @@ ctypedef const EVP_CIPHER * (* CIPHER)()
 
 
 cdef class _AEAD_BASE:
+    # new crypto used in borg >= 1.3
     # Layout: HEADER + MAC 16 + IV 12 + CT
 
     cdef CIPHER cipher