[JAVA][SPRING] add imports & primitives to reserved words

[martin-mfg]

In Java-based generators, add importMapping().keySet() and languageSpecificPrimitives() to reservedWords.

Because using words from those two lists as class names or variable names causes problems in certain cases.

This is a follow-up to #16276 (comment) . However, after deeper investigation, I deviated a bit from the approach that I originally proposed there.

The addition of more reserved words leads to some variables in the samples code being renamed, e.g. _integer and _dateTime. These didn't cause problems before the renaming actually, because the lowercase variable name "dateTime" doesn't clash with the uppercase class name "DateTime" anyway. But the current reserved words logic (of Java based generators) is case-independend and applies equally to variable names and class names. So in the current setup we can't avoid those unnecessary renamings I think. If you have an idea how to improve this situation, let me know.
Since this slightly changes some public method names in the generated code, I targeted this PR at the 8.0.x branch.
example:

-  public DataQuery date(OffsetDateTime date) {
+  public DataQuery _date(OffsetDateTime _date) {

Regarding the failing java-camel test, please see #18870 (comment) .

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[martin-mfg]

Hi @wing328, may I suggest adding this PR to https://github.com/OpenAPITools/openapi-generator/milestone/54 ? Just so it's not forgotten for the next major release. Unless you prefer not to include this in 8.0.0?