Do not reindex concurrently on PG < v16

[andyundso]

Fixes #67

System tables in database cannot be re-indexed concurrently on PG v15 and below. This PR switches to the reindexdb command provided by Postgres to start the reindex operation, and depending on the version, activates CONCURRENTLY or not.

Without CONCURRENTLY, Postgres locks the database during the INDEX operation. I added an information about that to the README.

[justinclift]

Awesome, this seems like a good approach. 😄

The only thing that stands out as a bit unusual to me is the use of bc and a subshell for the value comparison:

$(echo "15 <= $PGTARGET" | bc)

Wouldn't the more standard "$PGTARGET" -le 15 do the same thing and not need bc installed?

[andyundso]

I am not the greatest bash programmer, but I am learning :)

removed bc and adjusted the version check according to your suggestion @justinclift in the latest commit.

[justinclift]

Cool, that looks good. 😄