Handle pre-3.0 specVersions in MultiFormatStore serialize guard.

[tiegz]

In 42f69e2 a guard was added against the specVersion while serializing. It checks ">= '3.0'", but before 3.0 specVersion values looked like "SPDX-2.3" (correct me if I'm wrong). This adds a regression test and fix for the older versions.

This fixes a failure we ran into after upgrading to 2.0.5 of the library:

abc.def.TransformationException: Unable to serialize SPDX JSON for https://abc.def/spdx/some-project
      at app//abc.def.Spdx23SbomGenerator.serializeSbom(Spdx23SbomGenerator.java:87)

      Caused by:
      org.spdx.core.InvalidSPDXAnalysisException: Attempting to serialize an SPDX Spec version 3 model object
          at app//org.spdx.jacksonstore.MultiFormatStore.serialize(MultiFormatStore.java:186)
          at app//abc.def.Spdx23SbomGenerator.serializeSbom(Spdx23SbomGenerator.java:83)
          ... 2 more

[goneall]

@tiegz - I think you're right on this. I'm not sure how this made it past the unit tests. I'm going to do a bit of investigation on why the unit tests didn't catch this before merging.

[goneall]

Looks like I didn't have this particular method in the test cases

Thanks @tiegz for the PR!