BP-69: Convert stats and allocator modules to slog (phase 2)

[merlimat]

Part of the slog migration for BP-69. Tracking issue: #4750.

This PR stands alone — it does not depend on the bookkeeper-common PR (#4754) landing first. The 2 commits are:

1. 526e9ac7ec BP-69 base: add slog dependency and LICENSE entries — minimal scaffolding:
   • Add io.github.merlimat.slog:slog:0.9.7 to pom.xml (alongside SLF4J; SLF4J stays as the rendering backend).
   • Add lombok.config so @CustomLog generates a slog Logger.
   • Register the slog jar in LICENSE-all.bin.txt, LICENSE-server.bin.txt, LICENSE-bkctl.bin.txt.
2. db52607eb4 Convert stats and allocator modules from SLF4J to slog — the actual conversion (9 files):
   • stats/bookkeeper-stats-api: AlertStatsLogger, Stats, ThreadRegistry.
   • stats/bookkeeper-stats-providers: CodahaleMetricsProvider, OtelMetricsProvider, PrometheusMetricsProvider, ThreadScopedDataSketchesStatsLogger.
   • stats/utils/StatsDocGenerator.
   • bookkeeper-common-allocator/ByteBufAllocatorImpl.

If the bookkeeper-common PR (#4754) lands first, this PR will rebase cleanly onto it (the base commit here is a subset of what #4754 already contains, and will become empty on rebase).

Conventions applied

• @Slf4j → @CustomLog; LoggerFactory.getLogger(X.class) → Logger.get(X.class); static = LOG, instance = log.
• log.info("text {} {}", a, b) → log.info().attr("a", a).attr("b", b).log("text").
• log.error("msg", throwable) → log.error().exception(throwable).log("msg").
• if (log.isDebugEnabled()) guards removed.

Notes

• CodahaleMetricsProvider retains one org.slf4j.LoggerFactory reference for the Dropwizard Slf4jReporter.outputTo() call (that API takes an SLF4J Logger); the class's own logs are on slog.
• AlertStatsLogger.raise(String, Object...) keeps its vararg signature in this PR. The Map-based API change (raise(String, Map<String, Object>)) will land together with the stream/distributedlog conversion so the two sides stay in sync.

Not included

• No wire protocol, binary format, metadata format, metric, or CLI change.
• No module outside stats/* + bookkeeper-common-allocator is converted.

Test plan

• mvn -pl stats/...,bookkeeper-common-allocator -am compile -DskipTests passes
• Existing stats/* and bookkeeper-common-allocator unit tests still pass
• Alert logs from AlertStatsLogger continue to render correctly under Logback/Log4j2