[bitnami/postgresql-ha] fix(backup): mount secrets with password file

[fmulero]

Description of the change

Mount secret with postgres password in the backup job.

Benefits

Backup job can work properly with usePasswordFiles enabled.

Possible drawbacks

Users with a custom backup.cronjob.command value, will need to adapt their scripts.

Applicable issues

• fixes Built in backup script broken by :"Set usePasswordFiles=true by default #32115" #32959

Additional information

Checklist

• Chart version bumped in Chart.yaml according to semver. This is not necessary when the changes only affect README.md files.
• Variables are documented in the values.yaml and added to the README.md using readme-generator-for-helm
• Title of the pull request follows this pattern [bitnami/<name_of_the_chart>] Descriptive title
• All commits signed off and in agreement of Developer Certificate of Origin (DCO)