Fix checkpoint parsing on Postgres 18

[nuno-faria]

Postgres 18 changed the way checkpoint messages are logged:

Add column pg_stat_checkpointer.slru_written to report SLRU buffers written (Nitin Jadhav) §
Also, modify the checkpoint server log message to report separate shared buffer and SLRU buffer values.

This causes the parsing to fail, showing charts with the NO DATASET warning.

This PR simply updates the checkpoint regex to support both the older wrote X buffers (Y%); as well as the newer wrote X buffers (Y%), wrote Z SLRU buffers;.