docs: recommend SHA-256 for new OAEP usage

[salasebas]

Addresses #906.

Summary

This keeps the RSAES-OAEP default hash algorithm as SHA-1 for compatibility with RFC 8017 and existing ciphertexts, but updates the documentation to recommend SHA-256 explicitly for new protocols.

Changes included:

• Document that SHA-1 remains the default because it is the RFC 8017 RSAES-OAEP default.
• Recommend SHA-256 or stronger hashes for new OAEP usage when legacy interoperability does not require SHA-1.
• Update OAEP examples to pass hashAlgo=SHA256.
• Clarify the hashAlgo docstring in Crypto.Cipher.PKCS1_OAEP.

Rationale

Changing the default hash algorithm would silently break interoperability and decryption of existing OAEP ciphertexts created with the current default. A documentation/example update gives new users safer guidance without introducing a backward compatibility break.

Tests

• python3 setup.py build_ext -i
• env PYTHONPATH=lib:test_vectors python3 -c 'import unittest; from Crypto.SelfTest.Cipher import test_pkcs1_oaep; suite=unittest.TestSuite(test_pkcs1_oaep.get_tests({"slow_tests": False, "wycheproof_warnings": False})); result=unittest.TextTestRunner(verbosity=2).run(suite); raise SystemExit(0 if result.wasSuccessful() else 1)'
• git diff --check