Switch fuzzer workflow to PG18

Author: svenklemm

.github/workflows/libfuzzer.yaml

@@ -58,7 +58,7 @@ jobs:
       if: steps.cache-postgresql.outputs.cache-hit != 'true'
       run: |
        wget -q -O postgresql.tar.bz2 \
-         https://ftp.postgresql.org/pub/source/v${{ steps.config.outputs.PG15_LATEST }}/postgresql-${{ steps.config.outputs.PG15_LATEST }}.tar.bz2
+         https://ftp.postgresql.org/pub/source/v${{ steps.config.outputs.PG18_LATEST }}/postgresql-${{ steps.config.outputs.PG18_LATEST }}.tar.bz2
         mkdir -p ~/$PG_SRC_DIR
         tar --extract --file postgresql.tar.bz2 --directory ~/$PG_SRC_DIR --strip-components 1
         cd ~/$PG_SRC_DIR