Change klib PRE_RELEASE flag value to ignore it set in existing klibs

JsAllowValueClassesInExternals language feature had UNSTABLE_FEATURE
kind and was enabled by default for JS IR backend. As a result, klibs
compiled with recent compiler versions were marked with the pre-release
flag.

Now, if we enabled reading the flag from klibs, the JS IR compiler would
reject all these klibs by default. To fix that, this commit changes the
flag value from 0x1 to 0x2, so that all previously compiled klibs are
treated as not having the pre-release flag.

compiler/util-klib-metadata/src/org/jetbrains/kotlin/library/metadata/KlibMetadataHeaderFlags.kt

@@ -9,5 +9,6 @@ package org.jetbrains.kotlin.library.metadata
  * Possible values for [KlibMetadataProtoBuf.Header] flags field.
  */
 object KlibMetadataHeaderFlags {
-    const val PRE_RELEASE = 0x1
+    // Note: previously the value of this flag was 0x1.
+    const val PRE_RELEASE = 0x2
 }