sagemathgh-40675: Remove failing 'ubuntu-focal' from default CI platforms

It's failing since a couple of months, tracked at
sagemath#40305. Moreover, ubuntu-focal
has reached the end of its standard five-year support window on 31 May
2025.

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URL: sagemath#40675
Reported by: Tobias Diez
Reviewer(s): Dima Pasechnik

Author: Release Manager

.github/workflows/docker.yml

@@ -18,7 +18,6 @@ on:
         # 'tox -e update_docker_platforms' updates below
         default: >-
           [
-           "ubuntu-focal",
            "ubuntu-jammy",
            "ubuntu-noble",
            "debian-bullseye",

build/pkgs/configure/checksums.ini

@@ -1,3 +1,3 @@
 tarball=configure-VERSION.tar.gz
-sha1=62f8443779e60335448efaf4b9630e4a3c408b08
-sha256=5ffa599091a29f806868de85679ca146aa61a00c8c2068eb777174ab360b6d39
+sha1=c0842a0f9e137d93718b04ad4f637eaaaee398eb
+sha256=2d31f53a4597c431758db6ed62e8d15a7906a1f77f65ad9ac36c424d42ecc30e

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-510ca6a758c5b44b05df366d1223ece322fd474f
+13c1f3658226ccdc0c383da048972f8745b6be41