[BUG] SSLContext hardcoded to TLSv1.2 prevents TLS 1.3 connections in skip-verify and custom cert paths #76

Describe the bug

Rest and SslConfig both hardcode SSLContext.getInstance("TLSv1.2"), which caps TLS negotiation at 1.2 and prevents connections to servers that require TLS 1.3.

• Rest.DISABLED_SSL_CONTEXT (static initializer): used when sslVerification=false
• SslConfig.buildSslContextFromJks() / buildSslContextFromPem(): used when a custom trust store or PEM cert is configured.

Both should use SSLContext.getInstance("TLSv1.3") so the JVM negotiates the highest mutually supported version (1.2 or 1.3). Or at the very least support both TLSv1.2 and TLSv1.3 via SSLParameters(["TLSv1.2","TLSv1.3"]) since both TLSv1 and TLSv1.1 are considered insecure.

To Reproduce

1. Configure a server (e.g. HashiCorp Vault) with tls_min_version = "tls13"
2. Connect using vault-java-driver 6.x with either:

• SslConfig().verify(false) (skip SSL verification), or
• SslConfig().trustStoreFile(...) / SslConfig().pemFile(...) (custom CA cert)

3. Observe javax.net.ssl.SSLHandshakeException: (protocol_version) Received fatal alert: protocol_version

Expected behavior
Driver negotiates TLS 1.3 when the server requires it. Changing SSLContext.getInstance("TLSv1.2") to SSLContext.getInstance("TLS") in both locations restores standard JVM protocol negotiation (TLS 1.2 and 1.3).