tls: remove deprecated prefer_server_cipher_suites field (#23712)

The TLS configuration object includes a deprecated `prefer_server_cipher_suites`
field. In version of Go prior to 1.17, this property controlled whether a TLS
connection would use the cipher suites preferred by the server or by the
client. This field is ignored as of 1.17 and, according to the `crypto/tls`
docs: "Servers now select the best mutually supported cipher suite based on
logic that takes into account inferred client hardware, server hardware, and
security."

This property has been long-deprecated and leaving it in place may lead to false
assumptions about how cipher suites are negotiated in connection to a server. So
we want to remove it in Nomad 1.9.0.

Fixes: https://github.com/hashicorp/nomad-enterprise/issues/999
Ref: https://hashicorp.atlassian.net/browse/NET-10531

command/agent/testdata/basic.json

@@ -389,7 +389,6 @@
       "rpc_upgrade_mode": true,
       "tls_cipher_suites": "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256",
       "tls_min_version": "tls12",
-      "tls_prefer_server_cipher_suites": true,
       "verify_https_client": true,
       "verify_server_hostname": true
     }