ci: use Ubuntu 24.04 for legacy Python checks

The dependency `test (gitdb, 3.9)` and project
`install/import (ubuntu-latest, 3.9)` checks fail before installing code.
`ubuntu-latest` now selects Ubuntu 26.04, and `actions/setup-python` reports
that no Python 3.9 x64 build exists for that image.

Run the dependency matrix and the Linux legacy-runtime import checks on
Ubuntu 24.04. Preserve the import-check matrix names and the existing
macOS/Windows runners. The main modern-Python suite continues to cover the
current Ubuntu image.

Validated both workflow files as YAML and checked the upstream
`actions/python-versions` manifest for available Ubuntu 24.04 x64 builds of
Python 3.8, 3.9 and 3.10. This is a CI environment repair discovered while
validating the five issue-specific commits, not a runtime code change.

Author: codex

.github/workflows/dependencies.yml

@@ -11,7 +11,8 @@ permissions:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    # Python 3.8/3.9 builds are unavailable on Ubuntu 26.04.
+    runs-on: ubuntu-24.04
     strategy:
       fail-fast: false
       matrix:

.github/workflows/pythonpackage.yml

@@ -245,7 +245,8 @@ jobs:
 
   import-smoke:
     name: install/import (${{ matrix.os }}, ${{ matrix.python }})
-    runs-on: ${{ matrix.os }}
+    # Python 3.8/3.9 builds are unavailable on Ubuntu 26.04.
+    runs-on: ${{ matrix.os == 'ubuntu-latest' && 'ubuntu-24.04' || matrix.os }}
     strategy:
       fail-fast: false
       matrix: