Bump GEOS 3.9.1

[TomPohys]

Bump GEOS 3.9.1

postgres=# select postgis_full_version();
                                                                                  postgis_full_version                                                                                  
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 POSTGIS="3.1.1 aaf4c79" [EXTENSION] PGSQL="96" GEOS="3.9.1-CAPI-1.14.2" PROJ="Rel. 4.9.3, 15 August 2016" LIBXML="2.9.4" LIBJSON="0.12.1" LIBPROTOBUF="1.2.1" WAGYU="0.5.0 (Internal)"

[TomPohys]

Some function of GEOS 3.9.1 will not be applicable (postgis/docker-postgis#233 (comment))

[nyurik]

Sigh, this is really beginning to feel like a hack - postgis is compiled against the old version, but then we swap geoslib to a new version underneath it, hoping that the lib is perfectly backwards compatible, which might not be the case... at the same time, manual compilation of postgis itself is even worse...

So let's continue with this patch for testing, but only keep it in master, or maybe release it as a side branch, but let's try to work with @ImreSamu on the upstream postgis docker image, and use that for releases

[TomPohys]

Yes, I agree to keep it in the master branch for just the latest docker image. After release Debian Bullseye with GEOS 3.9.0 we can remove the force GEOS update.