[MNG-6776] Inconsistent list of parameters for MojoDescriptor (#584)

[michael-o]

It may lead to confusion as parameters (list) and
parametersMap (map) may "fall apart" easily.

Also, parametersMap did not honor parameter
ordering while parameters list makes it look
that order is important.
Simply, rebuild the map always and retain
ordering as well.

Added UT and also removed some cruft provided
by java8.

---

https://issues.apache.org/jira/browse/MNG-7309

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[michael-o]

As a result of apache/maven-plugin-tools#253

[michael-o]

@slawekjaranowski This is a cherrypick from master. It does not change behavior. Unmodifiables should on main only and require a new issue. Note that this will need special handling in plugin tools otherwise it will fail.

[kwin]

LGTM now.

[slachiewicz]

good, maybe it's also worth to backport to 3.8.x?

[michael-o]

good, maybe it's also worth to backport to 3.8.x?

I was thinking about the same. Will prepare after this one.

[michael-o]

@cstamas Any objections before I backport this one?