postgis: backports of geos, proj and gdal added

apt.postgresql.org has so far not been shipping updated geos, proj and
gdal packages, but was using the versions provided by the respective
operating system version. This mean that some postgis features were
not available on all distributions.

I have now added backports of geos, proj and gdal to apt.pg.o. On
Debian trixie, the `select postgis_full_version();` changes from

POSTGIS="3.6.4 94d984b" [EXTENSION] PGSQL="180" GEOS="3.13.1-CAPI-1.19.2" PROJ="9.6.0 NETWORK_ENABLED=OFF URL_ENDPOINT=https://cdn.proj.org USER_WRITABLE_DIRECTORY=/tmp/proj DATABASE_PATH=/usr/share/proj/proj.db" (compiled against PROJ 9.6.0) GDAL="GDAL 3.10.3, released 2025/04/01" LIBXML="2.9.14" LIBJSON="0.18" LIBPROTOBUF="1.5.1" WAGYU="0.5.0 (Internal)" RASTER

to

POSTGIS="3.6.4 94d984b" [EXTENSION] PGSQL="180" GEOS="3.14.1-CAPI-1.20.5" PROJ="9.8.1 NETWORK_ENABLED=OFF URL_ENDPOINT=https://cdn.proj.org USER_WRITABLE_DIRECTORY=/tmp/proj DATABASE_PATH=/usr/share/proj/proj.db" (compiled against PROJ 9.8.1) GDAL="GDAL 3.13.1 "Iowa City", released 2026/06/01" LIBXML="2.9.14" LIBJSON="0.18" LIBPROTOBUF="1.5.1" WAGYU="0.5.0 (Internal)" RASTER

Nothing is expected to break, but we'll only put this change live with
the next postgis update (likely the upcoming 3.7.0 release). If you
want to test before that, add xxx-pgdg-testing to your sources.list.

Christoph