[REQ] Generated date should be reproducible (or disabled by default)

Is your feature request related to a problem? Please describe.

To meet the requirements of Reproducible Builds, generated timestamps should be predicable (so that packages can be rebuild with the same source and produce the same output)

Describe the solution you'd like

For projects using the Maven plugin, this could mean that the generatedDate would be set to Maven's project.build.outputTimestamp property.

https://github.com/OpenAPITools/openapi-generator/blob/6da343a8ad1176875f27e7a4465164646574dc3d/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultGenerator.java#L289

Gradle's approach to this is to use a fixed date:
https://github.com/gradle/gradle/blob/f4fcd0b4f2c6a1903f80a4299a540d73d722f127/platforms/core-runtime/files/src/main/java/org/gradle/api/internal/file/archive/ZipEntryConstants.java#L24-L39

Describe alternatives you've considered

Alternatively configOptions.hideGenerationTimestamp could be set to true by default.

Another alternative would be to use the timestamp of the openapi spec file. There are a lot of edge cases around this, the file could be downloaded and timestamp not preserved, file could be copied at build time (and timestamp not preserved). Additionally, other inputs to the generator could change resulting in different code using the same source spec. So this probably isn't a valid option.

Additional context

See:

• Maven Artifact Plugin (list of non-compliant and fixed plugin versions)
• Maven Reproducible Builds Guide
• Maven Reproducible Builds wiki page

Start at modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultGenerator.java around line 289 and trace how generatedDate is set for Maven plugin users. Compare using Maven's project.build.outputTimestamp with disabling the timestamp by default, then define and verify behavior where repeated builds from the same inputs produce identical output.