Replace deprecated use of `toBytes()` with `Character` property `.code` (#18236)

Addresses the warning which is otherwise output during build asking that we do this.

Closes #18236

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/18236 from tempoz:tempoz-fix-deprecated-use-of-to-bytes bb2398c45d
PiperOrigin-RevId: 673844589

java/kotlin/src/test/kotlin/com/google/protobuf/ByteStringsTest.kt

@@ -35,8 +35,8 @@ class ByteStringsTest {
   @Test
   fun byteAt() {
     val str = "abc".toByteStringUtf8()
-    assertThat(str[0]).isEqualTo('a'.toByte())
-    assertThat(str[2]).isEqualTo('c'.toByte())
+    assertThat(str[0]).isEqualTo('a'.code)
+    assertThat(str[2]).isEqualTo('c'.code)
   }
 
   @Test