Move Docker base image to Debian trixie for Java 21

Planetiler 0.10.2 needs Java 21+; bookworm's default-jre-headless (and
its backports) only goes up to 17, causing a real build to fail with
UnsupportedClassVersionError. trixie's default-jre-headless is Java 21.

docs/third-party-notices.adoc

@@ -134,7 +134,7 @@ terrain.pmtiles regeneration.
 | https://github.com/onthegomap/planetiler
 
 | osmium-tool (`osmium`, installed via the `osmium-tool` Debian package)
-| (Debian bookworm's packaged version)
+| (Debian trixie's packaged version)
 | GPL-3.0
 | https://github.com/osmcode/osmium-tool
 |===
@@ -144,5 +144,5 @@ To update a vendored library: download its package (e.g.
 `public/vendor/<name>/`, and update the version in this file. To
 update go-pmtiles or Planetiler, bump `PMTILES_VERSION`/
 `PLANETILER_VERSION` in the `Dockerfile` and the version here —
-osmium-tool tracks whatever version Debian bookworm currently packages,
+osmium-tool tracks whatever version Debian trixie currently packages,
 nothing to pin on this side.