Migrate stdlib, tests and samples to new case conversion api

libraries/stdlib/test/text/StringTest.kt

@@ -290,6 +290,7 @@ class StringTest {
 
     @Test fun capitalize() {
         fun testCapitalize(expected: String, string: String) {
+            @Suppress("DEPRECATION")
             assertEquals(expected, string.capitalize())
             assertEquals(expected, string.replaceFirstChar { it.uppercase() })
         }
@@ -301,6 +302,7 @@ class StringTest {
 
     @Test fun decapitalize() {
         fun testDecapitalize(expected: String, string: String) {
+            @Suppress("DEPRECATION")
             assertEquals(expected, string.decapitalize())
             assertEquals(expected, string.replaceFirstChar { it.lowercase() })
         }