main/openssh: set license to proper SPDX identifier
CI linted that "BSD" is not a valid SPDX identifier. OpenSSH uses a bespoke license that SPDX recognizes as "SSH-OpenSSH", so set the license to that.
CI linted that "BSD" is not a valid SPDX identifier. OpenSSH uses a bespoke license that SPDX recognizes as "SSH-OpenSSH", so set the license to that.