create_certs.sh: Unify RSA and ML-DSA-65 SSL cert generation

[junaruga]

Merge create_mldsa65_certs.sh into create_certs.sh, removing duplicated logic
between create_certs.sh and create_mldsa65_certs.sh files.

• Create create_ssl_certs function with genpkey_args and prefix
  arguments to manage common logic between RSA and ML-DSA-65.
• Use "openssl genpkey" instead of "openssl genrsa" for RSA key generation.
• Don't use openssl.cnf any more.
• Create temporary files under tmp/create_certs/
• Remove tool/create_mldsa65_certs.sh as its logic is now in
  create_certs.sh

The 1st commit is the refactoring for the create_mldsa65_certs.sh and create_certs.sh.
The 2nd commit is to upgrade SSL/TLS RSA test/rubygems/*.pem files generated by new create_certs.sh.

Note that previous "openssl ca" commands did output human-readable text + PEM format.
With new openssl commands, the human readable texts are removed.
This is not a problem.

test/rubygems/ca_cert.pem
test/rubygems/client.pem
test/rubygems/ssl_cert.pem

Assisted-by: Claude:claude-opus-4-6[1m]

What was the end-user or developer problem that led to this PR?

tool/create_certs.sh and tool/create_mldsa65_certs.sh has duplicated logic for each other. This situation is not convenient when managing the files and adding new algorithms.

What is your fix for the problem, implemented in this PR?

Merged tool/create_mldsa65_certs.sh into tool/create_certs.sh, removing duplicated logic.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[junaruga]

Note that I confirmed that RSA 2048 bits CA/Server/Client certificate and key files by the following openssl commands.

$ openssl x509 -in test/rubygems/ca_cert.pem -noout -text | grep Algorithm
        Signature Algorithm: sha256WithRSAEncryption
            Public Key Algorithm: rsaEncryption
    Signature Algorithm: sha256WithRSAEncryption
    
$ openssl x509 -in test/rubygems/ssl_cert.pem -noout -text | grep Algorithm
        Signature Algorithm: sha256WithRSAEncryption
            Public Key Algorithm: rsaEncryption
    Signature Algorithm: sha256WithRSAEncryption
    
$ openssl x509 -in test/rubygems/client.pem -noout -text | grep Algorithm
        Signature Algorithm: sha256WithRSAEncryption
            Public Key Algorithm: rsaEncryption
    Signature Algorithm: sha256WithRSAEncryption

$ openssl pkey -in test/rubygems/ssl_key.pem -noout -text | head -1
Private-Key: (2048 bit, 2 primes)

[junaruga]

@hsbt This PR is ready to be reviewed. Could you review the PR? Thank you.