azdev extension create generates a azext_metadata.json that with old maxCliCoreVersion

When creating new extensions: `azdev extension create` you are left with an extension that won't work with current releases of the azure cli because of out of data [azext_metadata.json](https://github.com/Azure/azure-cli-dev-tools/blob/dev/azdev/mod_templates/azext_metadata.json).


[azext_metadata.json](https://github.com/Azure/azure-cli-dev-tools/blob/dev/azdev/mod_templates/azext_metadata.json) needs to be updated to match more [current releases](https://github.com/Azure/azure-cli/releases) of the azure cli.