UnderlineNav: Display loading counters only for items with the "counter" prop (#4745)

* shows LoadingCounter only when counter prop is given

* shows LoadingCounter only when counter prop is given

* test(vrt): update snapshots

* Update .changeset/erken-gelen-oturur.md

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Co-authored-by: Katie Langerman <18661030+langermank@users.noreply.github.com>
Co-authored-by: Josh Black <joshblack@users.noreply.github.com>
Co-authored-by: Armağan <broccolinisoup@github.com>

Author: 4 people

.changeset/erken-gelen-oturur.md

@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+UnderlineNav: Display loading counters only for items with the "counter" prop.