Add disableUpstreamHealthcheck configuration value

APISIX 3.14.0 has added a configuration option in `config.yaml` to disable all upstream health check:

```yaml title="config.yaml"
apisix:
disable_upstream_healthcheck: true
```

https://apisix.apache.org/blog/2025/10/10/release-apache-apisix-3.14.0/#add-global-switch-to-disable-all-upstream-health-checks

Helm chart should add this configuration value.

Use the APISIX 3.14.0 config.yaml example as the reference, then inspect the Helm chart's existing values and configuration templates for similar options. Done means the disable_upstream_healthcheck value can be set through the chart and appears correctly in the rendered APISIX configuration.

Written by the indexing model from the issue text.