drpc,security: DRPC certificate validation issue in TestUseWrongSplitCACerts

During the gRPC to DRPC migration work in [PR #152016](https://github.com/cockroachdb/cockroach/pull/152016), we discovered a certificate validation issue in the `TestUseWrongSplitCACerts` test that requires DRPC to be disabled

The test `TestUseWrongSplitCACerts` in `pkg/security/certs_test.go` removes `ca-client.crt` and `ca-ui.crt` to test fallback to `ca.crt`. However, the client certificates (`client.node.crt`, `client.root.crt`) were signed by the removed `ca-client.crt`. When DRPC tries to establish connections during startup for internal APIs, it fails with "tls: unknown certificate authority".

This is a test infrastructure issue, not a production concern, as production deployments don't remove CA certificates after generating client certificates. However, it blocks enabling DRPC for this specific test.

Priority: Low - This is a test infrastructure issue that doesn't affect production functionality

Jira issue: CRDB-54465

Start in pkg/security/certs_test.go at TestUseWrongSplitCACerts and review the DRPC startup path involved in the test. Reproduce the failure and inspect how ca-client.crt, ca-ui.crt, ca.crt, client.node.crt, and client.root.crt are used. Done means the fallback test runs without the unknown certificate authority error while preserving its certificate coverage.

Written by the indexing model from the issue text.