Use Charset instead of String for Mustache templates support

The MustacheResourceTemplateLoader previously defined the template
encoding as a String, defaulting to "UTF-8". This change replaces the
field with a Charset and initializes it with StandardCharsets.UTF_8.

Using Charset improves type safety and aligns with modern Spring Boot
standards, while avoiding implicit charset lookup issues.

See spring-projectsgh-48347

Signed-off-by: djlee <ddongjunn@gmail.com>

Author: ddongjunn

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

@@ -124,6 +124,12 @@ public Charset getCharset() {
 		return this.charset;
 	}
 
+	/**
+	 * Get the charset name.
+	 * @return the charset name
+	 * @deprecated since 4.1.0 in favor of {@link #getCharset()}
+	 */
+	@Deprecated(since = "4.1.0")
 	public String getCharsetName() {
 		return this.charset.name();
 	}

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

@@ -18,6 +18,8 @@
 
 import java.io.InputStreamReader;
 import java.io.Reader;
+import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
 
 import com.samskivert.mustache.Mustache;
 import com.samskivert.mustache.Mustache.Compiler;
@@ -45,7 +47,7 @@ public class MustacheResourceTemplateLoader implements TemplateLoader, ResourceL
 
 	private String suffix = "";
 
-	private String charSet = "UTF-8";
+	private Charset charSet = StandardCharsets.UTF_8;
 
 	private ResourceLoader resourceLoader = new DefaultResourceLoader(null);
 
@@ -60,8 +62,19 @@ public MustacheResourceTemplateLoader(String prefix, String suffix) {
 	/**
 	 * Set the charset.
 	 * @param charSet the charset
+	 * @deprecated since 4.1.0 in favor of {@link #setCharset(Charset)}
 	 */
+	@Deprecated(since = "4.1.0")
 	public void setCharset(String charSet) {
+		this.charSet = Charset.forName(charSet);
+	}
+
+	/**
+	 * Set the charset.
+	 * @param charSet the charset
+	 * @since 4.1.0
+	 */
+	public void setCharset(Charset charSet) {
 		this.charSet = charSet;
 	}