wrong proto3 schema default values for enums

per proto3 schema definition, enum values of `0` are, per definition, used as "unspecified" or "fallback" values.

see https://protobuf.dev/programming-guides/enum/
> ```proto3
> enum Enum {
> A = 0;
> B = 1;
> }
>
> message Msg {
> optional Enum enum = 1;
> }
> ```
>
> Closed enums will parse the value `2` and store it in the message’s unknown field set. Accessors will report the field as being unset and will return the enum’s default value.

Therefore, enum value of `0` MUST be revisited.

The CDX v1.x protobuf schema has default values for enums, where XML/JSON does not.
And this is for the fact, that protobuf schema uses value `0`, where it should not.

When these enum values would be changed, this would be a breaking change.

This is a followup of https://github.com/CycloneDX/specification/pull/385

----

examples for wrong usage of `0` in enums:
- `EvidenceTechnique` defaults to `EVIDENCE_TECHNIQUE_SOURCE_CODE_ANALYSIS`
https://github.com/CycloneDX/specification/blob/8af880d5f2ba0a107de88a920a76cedd5ba75083/schema/bom-1.5.proto#L720-L731
- `ModelParameterApproachType` defaults to `MODEL_PARAMETER_APPROACH_TYPE_SUPERVISED`
https://github.com/CycloneDX/specification/blob/8af880d5f2ba0a107de88a920a76cedd5ba75083/schema/bom-1.5.proto#L1096-L1102
- `ComponentDataType` defaults to `COMPONENT_DATA_TYPE_SOURCE_CODE`
https://github.com/CycloneDX/specification/blob/8af880d5f2ba0a107de88a920a76cedd5ba75083/schema/bom-1.5.proto#L1150-L1161