Change default file extension for Mustache templates

This commit changes the default file extension for Mustache templates,
from `.html` to `.mustache`, which is the file extension used in the
official reference documentation and by most IDE plugins.

Fixes gh-8997

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/mustache/MustacheProperties.java

@@ -30,7 +30,7 @@ public class MustacheProperties extends AbstractTemplateViewResolverProperties {
 
 	public static final String DEFAULT_PREFIX = "classpath:/templates/";
 
-	public static final String DEFAULT_SUFFIX = ".html";
+	public static final String DEFAULT_SUFFIX = ".mustache";
 
 	/**
 	 * Prefix to apply to template names.

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -372,7 +372,7 @@ content into your application; rather pick only the properties that you need.
 	spring.mustache.expose-spring-macro-helpers= # Set whether to expose a RequestContext for use by Spring's macro library, under the name "springMacroRequestContext".
 	spring.mustache.prefix=classpath:/templates/ # Prefix to apply to template names.
 	spring.mustache.request-context-attribute= # Name of the RequestContext attribute for all views.
-	spring.mustache.suffix=.html # Suffix to apply to template names.
+	spring.mustache.suffix=.mustache # Suffix to apply to template names.
 	spring.mustache.view-names= # White list of view names that can be resolved.
 
 	# SPRING MVC ({sc-spring-boot-autoconfigure}/web/WebMvcProperties.{sc-ext}[WebMvcProperties])