fix: update go release version for plugin validator

[helenosheaa]

the grafana plugin validator only works against go 1.21

Run git clone https://github.com/grafana/plugin-validator
Cloning into 'plugin-validator'...
~/work/grafana-flightsql-datasource/grafana-flightsql-datasource/plugin-validator/pkg/cmd/plugincheck2 ~/work/grafana-flightsql-datasource/grafana-flightsql-datasource
go: errors parsing go.mod:
/home/runner/work/grafana-flightsql-datasource/grafana-flightsql-datasource/plugin-validator/go.mod:3: invalid go version '1.21.8': must match format 1.23
/home/runner/work/grafana-flightsql-datasource/grafana-flightsql-datasource/plugin-validator/go.mod:5: unknown directive: toolchain