Convert grafana-auth-secret to use Grafana chart values rather than monitoring values

https://repo1.dso.mil/big-bang/bigbang/-/blob/master/chart/templates/grafana/flux/grafana-auth-secret.yaml?ref_type=heads

This file appears to be moved erroneously from the monitoring folder to the grafana folder, and should be updated to reflect new chart values rather than old ones

reported by Jon Hutches BB slack https://bigbanguniver-ft39451.slack.com/archives/C051A2BPS0K/p1761247207688669