Update postgres to more recent version for mattermost

Summary

Update postgres to more recent version for mattermost. any version currently maintained in ironbank will work. Recommend we use ironbank/opensource/postgres/postgresql:15

https://repo1.dso.mil/big-bang/product/packages/mattermost/-/blob/main/chart/values.yaml?ref_type=heads#L229

added Big Bang Add-Ons mattermost labels

changed the description

set weight to 2

changed iteration to Big Bang Iterations Jun 25, 2024 - Jul 8, 2024

added priority1 label

added teamStorage & Collaboration label

added kindmaintenance label

assigned to @akesterson

added statusdoing label

mentioned in commit 62c6cf1c

mentioned in merge request !243 (merged)

mentioned in commit c29315e4

added statusreview label and removed statusdoing label

mentioned in issue #156 (closed)

mentioned in commit 79166f47

mentioned in merge request !246 (merged)

mentioned in commit 68d41adc

mentioned in commit 25fa07b4

mentioned in commit 547170ad

mentioned in commit 617a3cf0