Switch to load_config in httpbin demo charm

[dwilding]

To keep our example charms fresh on the latest Ops features, we should update the httpbin charm to use the upcoming load_config feature to validate the config option (as documented in Manage configuration).

For more details, see:

• https://github.com/canonical/operator/pull/1833#discussion_r2153217277
• https://github.com/canonical/operator/pull/1833#discussion_r2153220879

When working on this, don't forget to bump the minimum Ops version in requirements.txt.