Adjust mustache property name to match replacement

`spring.mustache.cache` should be replaced with
`spring.mustache.servlet.cache`, to avoid configuration warnings
on startup when using the properties migrator.

See gh-30774

spring-boot-project/spring-boot-devtools/src/main/resources/org/springframework/boot/devtools/env/devtools-property-defaults.properties

@@ -7,7 +7,7 @@ spring.freemarker.cache=false
 spring.graphql.graphiql.enabled=true
 spring.groovy.template.cache=false
 spring.h2.console.enabled=true
-spring.mustache.cache=false
+spring.mustache.servlet.cache=false
 spring.mvc.log-resolved-exception=true
 spring.reactor.debug=true
 spring.template.provider.cache=false