[doc] Migration Guide for upgrading PMD 6 ➡️ 7

[adangel]

The following is done in #4590 :

• Differ between use cases, e.g.
  • I'm using only built-in rules -> check whether the ruleset/rules are still available,
    have the same properties, etc.
  • I'm using custom rules -> make sure to have good test coverage for your custom rules first.
    if XPath, need to migrate to XPath 2.0. if Java, some APIs/ASTs might have changed.
  • I've extended PMD with a custom language...
  • I've extended PMD with a custom feature...
• General approach: Update to the latest PMD 6.x version and try to fix deprecation warnings
• Compatibility Notes PMD 6 -> PMD 7:
  • Migrate rulechain rules:
    • RuleChain API changes, see [core] Simplify the rulechain #2490
    • addRuleChainVisit() ➡️ buildTargetSelector()
• CPD: ⚠: End Columns of Tokens are exclusive on PMD 7,
  but inclusive on PMD 6.x. See 5b7ed58
• Differences between XPath 1.0 and 2.0 (focused on practicality, eg how to transition)
  • This is already in place: Writing XPath rules - Migrating from 1.0 to 2.0
  • That means: The section migrating from XPath 1.0 -> XPath 2.0 should be moved to the migration guide PMD6->7
  • since only XPath 2.0 (actually XPath 3.1) will be supported with PMD 7.
• See also Java Clean Changes

Not done:

• Decide whether a migration tool from PMD 6 --> 7 is necessary
  • That depends on [[PMD 7.0.0 Documentation|PMD-7.0.0-Documentation]] / Migration Guide.
  • If something can be automated, it's maybe worth it. But maybe, we don't need it, if most of the simple stuff can be done already on PMD 6 by fixing deprecation warnings.

--> Decision: Migration tool won't be necessary, the guide is enough.