caddy: update to 2.7.6

In this version we've made several fixes and enhancements with help from
several contributors. Most changes are small, but some notable ones:

- The templates middleware is now officially extensible (experimentally). This
  means modules can add custom functions/actions for templates to execute.
- TLS storage cleaning is now synchronized across the cluster and remembered
  across restarts. This should greatly lower costs for expensive storage
  backends like DynamoDB.
- Placeholders are now evaluated in config for certificate loaders.
- Numerous bug fixes.

(bsiegert)