Update DefaultCharset.java

[delanym]

Prefer (?, Charset) to (?, String) constructor

Such as https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Scanner.html#%3Cinit%3E(java.io.File,java.nio.charset.Charset)

[cushon]

Looks good overall, thanks for the PR.

The tests are currently failing because they are assertion on the old version of the refactoring that included .name(), can you update them?

• error-prone/core/src/test/java/com/google/errorprone/bugpatterns/DefaultCharsetTest.java

  Lines 392 to 393 in 1147184

  	" PrintWriter pw3 = new PrintWriter(\"test\", UTF_8.name());",
  	" PrintWriter pw4 = new PrintWriter(new File(\"test\"), UTF_8.name());",

• error-prone/core/src/test/java/com/google/errorprone/bugpatterns/DefaultCharsetTest.java

  Lines 497 to 500 in 1147184

  	" new Scanner((InputStream) null, UTF_8.name());",
  	" new Scanner((File) null, UTF_8.name());",
  	" new Scanner((Path) null, UTF_8.name());",
  	" new Scanner((ReadableByteChannel) null, UTF_8.name());",