Bump PMD version to last planned release (6.55.0) (#178)

closes #176.

Currently shipped PMD version is at 6.46.0, released in May 2022.
Since then, a handful of fixes & enhancements got released, among which:
- pmd/pmd#3936 in 6.47.0
- pmd/pmd#4015 in 6.48.0
- pmd/pmd#4082 in 6.49.0
- pmd/pmd#4085 in 6.50.0
- pmd/pmd#4139 in 6.51.0
- pmd/pmd#4152 in 6.52.0
- pmd/pmd#4266 in 6.53.0
- pmd/pmd#4364 in 6.54.0
- pmd/pmd#4333 in 6.55.0

The latter also
[states](https://docs.pmd-code.org/pmd-doc-6.55.0/pmd_release_notes.html#pmd-7-development)
that:
> This release is the last planned release of PMD 6. The first version
> 6.0.0 was released in December 2017. Over the course of more than 5
> years we published almost every month a new minor version of PMD 6
> with new features and improvements.

The present change therefore bumps the shipped PMD version to 6.55.0 so
as to benefit from its last improvements, before envisioning an upgrade
to PMD 7, [when ready](https://github.com/pmd/pmd/milestone/19).

WORKSPACE

@@ -71,7 +71,7 @@ maven_install(
         "com.puppycrawl.tools:checkstyle:10.2",
 
         # PMD deps
-        "net.sourceforge.pmd:pmd-dist:6.46.0",
+        "net.sourceforge.pmd:pmd-dist:6.55.0",
 
         # Spotbugs deps
         # We don't want to force people to use 1.8-beta