Clarify icon sizing rules for NFS icons

Document the IconElement sizing contract, ensure deprecated icon component registrations inherit size correctly, and add changesets for the affected icon migration packages.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

.changeset/nfs-icon-alpha-plugins.md

@@ -0,0 +1,13 @@
+---
+'@backstage/plugin-api-docs': patch
+'@backstage/plugin-app-visualizer': patch
+'@backstage/plugin-catalog': patch
+'@backstage/plugin-catalog-unprocessed-entities': patch
+'@backstage/plugin-devtools': patch
+'@backstage/plugin-scaffolder': patch
+'@backstage/plugin-search': patch
+'@backstage/plugin-techdocs': patch
+'@backstage/plugin-user-settings': patch
+---
+
+Updated alpha plugin icons to follow the new frontend icon sizing rules when rendered in plugin and navigation surfaces.

.changeset/nfs-icon-foundations.md

@@ -0,0 +1,7 @@
+---
+'@backstage/core-components': patch
+'@backstage/frontend-app-api': patch
+'@backstage/frontend-plugin-api': patch
+---
+
+Clarified the `IconElement` sizing contract for the new frontend system and aligned legacy system icon rendering with the new icon API.