chore(speedtest-tracker): set data retention to 30 days

compose/speedtest-tracker/docker-compose.yml

@@ -16,6 +16,7 @@ services:
       DISPLAY_TIMEZONE: "${TZ:-Etc/UTC}"
       DATETIME_FORMAT: "${DATETIME_FORMAT:-j M Y, G:i:s}"
       SPEEDTEST_SCHEDULE: "${SPEEDTEST_SCHEDULE:-0 */2 * * *}"
+      PRUNE_RESULTS_OLDER_THAN: "30"
     volumes:
       - "${CONFIG_PATH:?}:/config"
       - "${KEYS_PATH:?}:/config/keys"