gh-39164: Update conda to use Python 3.12, drop 3.9 and 3.10
    
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In line with Spec 0, we drop Python 3.9 and 3.10 support (in this PR
only for conda). Instead Python 3.11 and 3.12 is used for the CI.

(Python 3.13 still needs a few package updates in the conda universe
#39163)

### 📝 Checklist

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- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
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### ⌛ Dependencies

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URL: #39164
Reported by: Tobias Diez
Reviewer(s): Dima Pasechnik

Author: Release Manager

.github/workflows/ci-conda.yml

@@ -26,23 +26,12 @@ jobs:
         # On pushes to tags or branches, test the whole matrix.
         os: >-
           ${{ github.event_name == 'pull_request'
-                && fromJson('["ubuntu-latest"]')
+                && fromJson('["ubuntu-latest", "macos-latest"]')
                 || fromJson('["ubuntu-latest", "macos-latest", "macos-13"]') }}
-        python: >-
-          ${{ github.event_name == 'pull_request'
-                && fromJson('["3.9"]')
-                || fromJson('["3.9", "3.10", "3.11"]') }}
+        python: ['3.11', '3.12']
         # Optional environment is disabled for now as its not yet working
         # environment: [environment, environment-optional]
         conda-env: [environment]
-        # On pull requests, only test two jobs:
-        # Ubuntu with Python 3.9, macOS (arm64) with Python 3.11.
-        # Build & Test currently uses Python 3.10 (on ubuntu-jammy).
-        # Together, they cover the supported minor Python versions.
-        include: >-
-          ${{ github.event_name == 'pull_request'
-                && fromJson('[{"os": "macos-latest", "python": "3.11", "conda-env": "environment"}]')
-                || fromJson('[]') }}
 
     steps:
       - uses: actions/checkout@v4

.github/workflows/ci-meson.yml

@@ -23,7 +23,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu]
-        python: ['3.9', '3.10', '3.11']
+        python: ['3.11', '3.12']
 
     steps:
       - uses: actions/checkout@v4

.gitignore

@@ -26,13 +26,11 @@
 # no longer generated, but may still be in user worktrees
 /src/lib/pkgconfig
 
-# Conda environment files
-# The files without Python version, with -dev or in src are no longer generated
-# but may still be in users' directories.
+# Conda environment files (auto-generated)
+/environment-3.[0-9].yml
+/environment-3.[0-9][0-9].yml
+# The following files are no longer generated but may still be in users' directories
 /environment.yml
-/environment-3.9.yml
-/environment-3.10.yml
-/environment-3.11.yml
 /environment-dev-3.9.yml
 /environment-dev-3.10.yml
 /environment-dev-3.11.yml

Makefile

@@ -173,7 +173,7 @@ distclean: build-clean
 bootstrap-clean:
 	rm -rf config/install-sh config/compile config/config.guess config/config.sub config/missing configure build/make/Makefile-auto.in
 	rm -f src/doc/en/installation/*.txt
-	find src/doc/en/reference/spkg -name index.rst -prune -o -maxdepth 1 -name "*.rst" -exec rm -f {} \+
+	find src/doc/en/reference/spkg -maxdepth 1 -name index.rst -prune -o -name "*.rst" -exec rm -f {} \+
 	for a in environment environment-optional src/environment src/environment-optional; do rm -f $$a.yml $$a-3.[89].yml $$a-3.1[0-9].yml; done
 	rm -f src/requirements.txt
 	rm -f src/setup.cfg

bootstrap

@@ -224,8 +224,8 @@ save () {
 	config/install-sh config/compile config/config.guess config/config.sub config/missing \
 	build/make/Makefile-auto.in \
 	src/doc/en/installation/*.txt \
-	$(find src/doc/en/reference/spkg -name index.rst -prune -o -maxdepth 1 -name "*.rst" -print) \
-	environment-3.[89]-*.yml environment-3.1[0-9]-*.yml \
+	$(find src/doc/en/reference/spkg -maxdepth 1 -name index.rst -prune -o -name "*.rst" -print) \
+	environment-3.1[0-9]-*.yml \
 	src/pyproject.toml \
 	src/requirements.txt \
 	src/setup.cfg \