Composability updates for Splunk On-Call plugin

Signed-off-by: Tim Hansen <timbonicus@gmail.com>

.changeset/many-boats-cough.md

@@ -0,0 +1,23 @@
+---
+'@backstage/plugin-splunk-on-call': minor
+---
+
+Updates the Splunk On-Call plugin for the [composability system
+migration](https://backstage.io/docs/plugins/composability#porting-existing-plugins).
+
+To upgrade, modify your `EntityPage` to use the updated export names. The
+`EntitySplunkOnCallCard` should be wrapped in an `<EntitySwitch>` condition as
+shown in the plugin README, which you may already have in place.
+
+```diff
+import {
+- isPluginApplicableToEntity as isSplunkOnCallAvailable,
++  isSplunkOnCallAvailable,
+- SplunkOnCallCard
++  EntitySplunkOnCallCard,
+} from '@backstage/plugin-splunk-on-call';
+
+...
+-  <SplunkOnCallCard entity={entity}>
++  <EntitySplunkOnCallCard />
+```