Clean up Javadoc warnings and fail the build on new ones

[vharseko]

Summary

Makes the Javadoc build produce 0 warnings and turns warnings into hard build errors so regressions cannot slip back in — enforced across the whole JDK matrix (11, 17, 21, 25, 26).

Changes

Warning cleanup + enforcement

• Remove the deprecated <footer> config in the parent maven-javadoc-plugin setup — it only emitted the JDK -footer option is no longer supported and will be ignored warning (once per module). The <header> already carries the same title, so rendered docs are unchanged.
• Drop the empty <p></p> in launcheroverview.html that produced an empty <p> tag warning.
• HTML-escape &, <, > in the message text that the i18n AbstractGenerateMessagesMojo embeds into the Javadoc of generated message classes (LDAP filters / HTML fragments would otherwise emit invalid Javadoc in generated sources). The runtime message is still loaded by key from the property bundle, so escaping the doc comment is harmless.
• Enable failOnWarnings in maven-javadoc-plugin so any remaining Javadoc warning (html, syntax, reference, accessibility) fails the build. The missing doclint group stays suppressed via doclint=all,-missing, so documentation-completeness gaps do not fail the build — only real markup/reference problems do.
• Bump maven-javadoc-plugin to 3.12.0 (from 3.10.1) and align the two modules that pinned older versions (launcher 2.9, maven-external-dependency-plugin 2.6.1) to the shared ${javadocPluginVersion} property.

JDK 11 doclint: fill in empty block-tag descriptions

• JDK 11's doclint reports no description for @param/@throws/@return for block tags whose description is empty; JDK 17+ does not report this at all. With failOnWarnings on, that failed the JDK 11 CI jobs while JDK 17+ stayed green.
• Filled in concise, accurate descriptions for every such tag across the reactor — 370 tags in 90 files, overwhelmingly in the legacy persistit/core module (plus script, audit, json-schema, selfservice, launcher, maven-external-dependency-plugin). Descriptions are derived from each method's signature/body and reuse the module's house wording (e.g. PersistitException -> "if a persistence error occurs").
• These are comment-only changes — no code, signatures, or behavior touched. (Note: Javadoc prints at most 100 diagnostics per module, so the true count was far higher than the first ~100 visible; a static source scan was used to find and confirm every empty tag.)

Verification

• Static source scan across the whole reactor: 0 empty block tags remain.
• mvn javadoc:javadoc on JDK 11 (failOnWarnings=true) -> BUILD SUCCESS, 0 warnings.
• mvn javadoc:jar (the goal that failed CI) on JDK 11 -> BUILD SUCCESS.
• mvn javadoc:javadoc on JDK 26 -> BUILD SUCCESS, 0 warnings (confirms the new descriptions introduced no bad HTML/markup).
• Confirmed failOnWarnings actually bites: injecting an empty <p> / empty tag makes the build fail with Project contains Javadoc Warnings, then reverted.