build: bump linuxfabrik-lib pin from 3.0.0 to 3.1.0

Regenerate requirements.txt via pip-compile (Python 3.9 as
required by the RHEL 8 target) with
--upgrade-package=linuxfabrik-lib so only the linuxfabrik-lib
block changes; pip-tools 7.5 additionally collapses duplicate
hash lines that the previous compile left in the file, so the
diff is large but no other package versions are touched.

linuxfabrik-lib 3.1.0 ships the new lftest.run_mariadb() /
MARIADB_LTS_IMAGES container-test helpers and the attach_each()
/ attach_tests() unit-test helpers this repo already depends on
for its mysql-* and cpu-usage integration tests, plus a
disk.dir_exists() directory helper. No plugin code changes are
required alongside the bump.

Author: markuslf

CHANGELOG.md

@@ -65,6 +65,7 @@ Assets:
 
 Build, CI/CD:
 
+* Bump pinned `linuxfabrik-lib` dependency from 3.0.0 to 3.1.0, picking up the new `run_mariadb()` / `MARIADB_LTS_IMAGES` container-test helpers, the `attach_each()` / `attach_tests()` unit-test helpers and the `disk.dir_exists()` directory check
 * Windows MSI still installs all plugins to ProgramFiles64Folder/ICINGA2/sbin/linuxfabrik, but does not depend on an Icinga2 agent any longer