[maven-generator] Do not cleanup output if spec file is unchanged and skipIfSpecIsUnchanged=true (Fixes #16490)

[Philzen]

Closes #16490

[wing328]

@Philzen thanks for the PR. Have you tested it locally to ensure it's working for your use cases?

[wing328]

let's give it a try. thanks for the PR.

[Philzen]

@wing328 Apologies for not replying on this earlier. At the time of this PR, all the projects i was working on were still on Java 8 and Spring Boot 2.1, when i discovered this on my research regarding a migration path. I purely inferred the bug and its fix from looking at the code, hence i couldn't test this at the time.

Now the migration to SB 3.1 and openapitools 7.0.1 is in full swing and i was able to test thoroughly.
I am happy to confirm that #16490 was definitely a thing up to <= 7.0.0 and this fix did exactly what it was supposed to do.

For the record, release notes respectively, <cleanupOutput> was introduced in 4.0.2 and in 7.01 finally started to work smooth as silk in conjunction with skipIfSpecIsUnchanged.