[bitnami/mongodb]: Use merge helper

[juan131]

Description of the change

This PR follows up #18889 adapting the chart to use the new helper common.tplvalues.merge donated by @jouve to merge annotations & labels.

Benefits

Chart to be compatible with values with string templates & dicts, so both the formats below can be used:

podLabels:
  foo: "bar"
  app.kubernetes.io/name: "{{ join \"-\" (list \"prefix\" .Release.Name)  }}"

podLabels: |
  {{ include "XXX.labels" . }}

Possible drawbacks

None

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.
• 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)