doc: release: Finalize v3.7.0 release notes and migration guide

Remove working draft status from the 4.0.0 release notes and
migration guide.

Signed-off-by: Mahesh Mahadevan <mahesh.mahadevan@nxp.com>

doc/releases/migration-guide-4.0.rst

@@ -2,8 +2,8 @@
 
 .. _migration_4.0:
 
-Migration guide to Zephyr v4.0.0 (Working Draft)
-################################################
+Migration guide to Zephyr v4.0.0
+################################
 
 This document describes the changes required when migrating your application from Zephyr v3.7.0 to
 Zephyr v4.0.0.

doc/releases/release-notes-4.0.rst

@@ -2,8 +2,8 @@
 
 .. _zephyr_4.0:
 
-Zephyr 4.0.0 (Working Draft)
-############################
+Zephyr 4.0.0
+############
 
 We are pleased to announce the release of Zephyr version 4.0.0.