ci: modernize PostGIS test matrix for next

Test each supported Redmine version against the still-in-support Ruby versions
it allows (https://redmine.jp/tech_note/supported-rubies/; 3.1/3.2 are EOL):

- 6.0-stable -> Ruby 3.3
- 6.1-stable -> Ruby 3.3, 3.4
- Drop 5.1-stable (no currently supported Ruby) and master
- PostGIS (postgis/postgis): span PG 15/17/18 + PostGIS 3.4/3.5/3.6
  (13-3.4 & 17-3.5 -> 15-3.4, 17-3.5, 18-3.6)
- system_test row on 6.1-stable + Ruby 3.4
- Drop the now-dead 5.1-stable rgeo gem-version override step

Author: dkastl

.github/workflows/test-postgis.yml

@@ -25,17 +25,25 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        redmine_version: [5.1-stable, 6.0-stable, master]
-        ruby_version: ['3.1', '3.2', '3.3']
-        db_version: [13-3.4, 17-3.5]
+        # Test each Redmine version against the Ruby versions it supports
+        # (https://redmine.jp/tech_note/supported-rubies/), limited to Rubies
+        # still in upstream support (3.1 EOL 2025-03, 3.2 EOL 2026-03):
+        #   6.0-stable -> 3.3
+        #   6.1-stable -> 3.3, 3.4
+        # (5.1-stable dropped: no currently supported Ruby.)
+        redmine_version: [6.0-stable, 6.1-stable]
+        ruby_version: ['3.3', '3.4']
+        # postgis/postgis tags spanning PG 15/17/18 and PostGIS 3.4/3.5/3.6
+        # (floor -> ceiling; ceiling matches the PG18 + PostGIS 3.6 runtime).
+        db_version: [15-3.4, 17-3.5, 18-3.6]
         include:
           - system_test: true
-            redmine_version: 6.0-stable
-            ruby_version: '3.3'
+            redmine_version: 6.1-stable
+            ruby_version: '3.4'
         exclude:
-          - redmine_version: 5.1-stable
-            ruby_version: '3.3'
-          - redmine_version: master
+          # Redmine 6.0 supports Ruby up to 3.3 only.
+          - redmine_version: 6.0-stable
+            ruby_version: '3.4'
 
     services:
       postgres:
@@ -103,15 +111,6 @@ jobs:
               encoding: utf8
           EOF
 
-      - name: Adjust Gem environment
-        run: |
-          case "${{ matrix.redmine_version }}" in
-            5.1-stable)
-              echo "GEM_RGEO_ACTIVERECORD_VERSION=7.0.1" >> ${GITHUB_ENV}
-              echo "GEM_ACTIVERECORD_POSTGIS_ADAPTER_VERSION=7.1.1" >> ${GITHUB_ENV}
-              ;;
-          esac
-
       - name: Install Ruby dependencies
         working-directory: redmine
         run: |