[bitnami/thanos] Fix default value for ruler.alertmanagersconfig

[kir4h]

Description of the change

This PR changes the default value for ruler.alertmanagersConfig to be a map instead of a string for consistency with its documentation.

Benefits

When following the documentation, Helm will not issue the following warning when setting the variable.

coalesce.go:301: warning: destination for thanos.ruler.alertmanagersConfig is a table. Ignoring non-table value ()
coalesce.go:301: warning: destination for thanos.ruler.alertmanagersConfig is a table. Ignoring non-table value ()

Possible drawbacks

For users already using a string, a warning will now show up

coalesce.go:298: warning: cannot overwrite table with non table for thanos.ruler.alertmanagersConfig (map[])
coalesce.go:298: warning: cannot overwrite table with non table for thanos.ruler.alertmanagersConfig (map[])

Applicable issues

N/A

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)

[dgomezleon]

LGTM. Thanks for contributing!