Move default RSA encryption to RSA-

[simo5]

custodia/src/custodia/message/kem.py

Line 191 in 090115a

enc = ('RSA-OAEP', 'A256CBC-HS512')

The original commit that changed this from RSA1_5 mentioned not going immediately to RSA-OAEP-256 for compatibility reasons.
RSA-OAEP uses SHA-1 and we should move to SHA-256 to avoid questions as SHA-1 is being deprecated left and right.