[bitnami/memcached] Set usePasswordFiles=true by default

[migruiz4]

Description of the change

Sets default value for usePasswordFiles to true.

Benefits

With this change, the chart will mount the secrets as files by default instead of directly setting them into environment variables.

Possible drawbacks

None

Applicable issues

None

Additional information

N/A

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)