doc: release: v18.3.0: use semver combined fwbundle filenames

To be consistent with git tags, ensure that the combined firmware
bundle files produced with each release use semantic versioning
in the filename.

Signed-off-by: Chris Friedt <[email protected]>

Author: Chris Friedt

doc/release/migration-guide-18.3.0.md

@@ -5,3 +5,8 @@
 This document lists recommended and required changes for those migrating from the previous v18.2.0 firmware release to the new v18.3.0 firmware release.
 
 [comment]: <> (UL by area, indented as necessary)
+
+* Going forward, and following the more standard semantic versioning of firmware, the filename
+  of the combined firmware bundles will be of the form
+  * `fw_pack-<major>.<minor>.<patch>.fwbundle` for final releases, and
+  * `fw_pack-<major>.<minor>.<patch>-rc<N>.fwbundle` for release candidates (pre-releases)