Advance project version to 1.3-SNAPSHOT

Set LATEST_STABLE language version and current version of stdlib to 1.3, set IS_PRE_RELEASE
Remove "EXPERIMENTAL" from 1.3 version description in tests and gradle options

compiler/testData/cli/jvm/apiVersionInvalid.out

@@ -1,3 +1,3 @@
 error: unknown API version: 239.42
-Supported API versions: 1.0, 1.1, 1.2, 1.3 (EXPERIMENTAL), 1.4 (EXPERIMENTAL)
+Supported API versions: 1.0, 1.1, 1.2, 1.3, 1.4 (EXPERIMENTAL)
 COMPILATION_ERROR

compiler/testData/cli/jvm/languageVersionInvalid.out

@@ -1,3 +1,3 @@
 error: unknown language version: 239.42
-Supported language versions: 1.0, 1.1, 1.2, 1.3 (EXPERIMENTAL), 1.4 (EXPERIMENTAL)
+Supported language versions: 1.0, 1.1, 1.2, 1.3, 1.4 (EXPERIMENTAL)
 COMPILATION_ERROR

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -214,7 +214,7 @@ enum class LanguageVersion(val major: Int, val minor: Int) : DescriptionAware {
             str.split(".", "-").let { if (it.size >= 2) fromVersionString("${it[0]}.${it[1]}") else null }
 
         @JvmField
-        val LATEST_STABLE = KOTLIN_1_2
+        val LATEST_STABLE = KOTLIN_1_3
     }
 }