Fixed const issue with magick plugin

[bryancall]

When doing to QUIC builds for Github-autest I saw this issue:

The second argument should be a non const:

#  define EVP_PKEY_assign_RSA(pkey,rsa) EVP_PKEY_assign((pkey),EVP_PKEY_RSA,\
                                        (char *)(rsa))

Error from CI:

In file included from experimental/magick/magick.cc:39:0:
experimental/magick/magick.cc: In member function 'bool magick::EVPKey::assign(const char*) const':
experimental/magick/magick.cc:190:45: error: invalid conversion from 'const void*' to 'void*' [-fpermissive]
     const int rc = EVP_PKEY_assign_RSA(key, k);
/opt/openssl-quic/include/openssl/evp.h:418:42: note: in definition of macro 'EVP_PKEY_assign_RSA'
                                         (rsa))
                                          ^~~
/opt/openssl-quic/include/openssl/evp.h:1072:5: note:   initializing argument 3 of 'int EVP_PKEY_assign(EVP_PKEY*, int, void*)'
 int EVP_PKEY_assign(EVP_PKEY *pkey, int type, void *key);
     ^~~~~~~~~~~~~~~

[zwoop]

Cherry-picked to v9.0.x branch.

[maskit]

It seems like the same thing happens on PRs for 8.0.x.
Should we backport this to 8.0.x as well?

[randall]

8.0.x backport: #6041