fix: update maven boms and skip the artifacts they version

[dhalperi]

rules_jvm_external writes three files for @maven//:outdated --
outdated.repositories, outdated.artifacts and outdated.boms -- but
MavenDataExtractor only read the first two, so no bom was ever a
candidate for a bump.

Worse, declaring a bom is what strips versions off the artifacts it
manages, and those still appear in outdated.artifacts as
"group:artifact:" with an empty version. Those parsed into a
SimpleVersion(""), which toSemVer rejects, so UpdateLogic silently
dropped them. A repository that moves a family of artifacts under a bom
therefore stops getting updates for the bom and for every artifact the
bom versions.

Read outdated.boms as well, treating it as optional since it is absent
before rules_jvm_external 6.x, and filter versionless artifacts out
explicitly at parse time rather than relying on a null from toSemVer
several layers down. Bumping the bom is what moves the artifacts it
manages; they have no version text of their own to rewrite.

The e2e fixture pins rules_jvm_external 6.10 for boms support; the
existing 5.2 fixtures cover the absent-outdated.boms path.

[krakeusz]

Just one comment. Other than that, thank you for the contrubution!

[krakeusz]

The format seems to be group:artifact:version, so it seems you have provided no classifier in the test.
Could you rename the test? Or did you intend to test a group:artifact:packaging:classifier:version case?

[dhalperi]

You're right, no classifier there. coursier.bzl writes outdated.artifacts as group:artifact:version and strips the classifier first, so no 5-part case reaches this code.

I deleted the test instead of renaming it -- without the classifier its input was the same as should parse coordinates above it.