Replace the obsolete OpenSSL functions EVP_{De,En}cryptFinal (#757)
Replace the obsolete OpenSSL functions EVP_{De,En}cryptFinal (#757)

As noted by OpenSSL documentation [1]:

    The functions EVP_EncryptInit(), EVP_EncryptFinal(),
    EVP_DecryptInit(), EVP_CipherInit() and EVP_CipherFinal() are obsolete
    but are retained for compatibility with existing code. New code should
    use EVP_EncryptInit_ex(), EVP_EncryptFinal_ex(), EVP_DecryptInit_ex(),
    EVP_DecryptFinal_ex(), EVP_CipherInit_ex() and EVP_CipherFinal_ex()
    because they can reuse an existing context without allocating and
    freeing it up on each call.

[1]: https://www.openssl.org/docs/man1.1.0/man3/EVP_EncryptFinal.html#NOTES

These changes are already applied to the BoringSSL code.
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