Add pmd xml migration recipe

[sullis]

What's changed?

PMD 7 removed a batch of Java rules, and PMD fails on a <rule ref> it cannot resolve. So a PMD 6 ruleset that references any of them stops the build outright after upgrading to PMD 7, one unresolvable ref at a time.

This adds PmdXml6to7Migration, an XML recipe that updates a PMD ruleset file for PMD 7:

• repoints the 46 removed rules that have a single unambiguous PMD 7 successor at that successor — note the successor often lives in a different category than the rule it replaces, e.g. errorprone.xml/DataflowAnomalyAnalysis → bestpractices.xml/UnusedAssignment,
• drops the four rules removed without any successor (AvoidFinalLocalVariable, AvoidUsingShortType, CloneThrowsCloneNotSupportedException, SimplifyStartsWith), since leaving them behind is what breaks PMD 7,
• does not add a successor twice when several removed rules collapse onto one (all four UseAssert*InsteadOf* rules become SimplifiableTestAssertion) or when the ruleset already selects it,
• keeps nested <properties> on a rule it repoints.

<rule ref="category/java/errorprone.xml/DontImportSun" />
<rule ref="category/java/errorprone.xml/EmptyCatchBlock" />

becomes

<rule ref="category/java/errorprone.xml/EmptyCatchBlock" />
<rule ref="category/java/errorprone.xml/UnsupportedJdkApiUsage" />

A replacement is inserted in ref order relative to the rules already there, taking the indentation of its neighbor. Rules that aren't being replaced are never reordered, so a ruleset that happens to be unsorted stays as its author wrote it.

Two things are deliberately left alone:

• rules PMD 7 splits across several successors ({Boolean,Byte,Integer,Long,Short}Instantiation, MIsLeadingVariableName, VariableNamingConventions) — picking one successor would silently change what the ruleset covers, so this leaves the choice to a human,
• any XML document whose root isn't ruleset, rather than matching a ruleset tag that happens to appear elsewhere.

Also adds rewrite-xml to the build and registers the recipe in recipes.csv.

PMD project

https://docs.pmd-code.org/latest/index.html

https://github.com/pmd/pmd

Removed rules are listed in the PMD 7.0.0 release notes.