feat(catalog-backend): move filters into their own query key (#3066)

.changeset/3066.md

@@ -0,0 +1,12 @@
+---
+'@backstage/plugin-catalog-backend': minor
+---
+
+Filters passed to the `/entities` endpoint of the catalog has changed format.
+
+The old way was to pass things on the form `?a=b&c=d`; the new way is to pass
+things on the form `?filter=a=b,c=d`. See discussion in
+[#2910](https://github.com/spotify/backstage/issues/2910) for details.
+
+The comma separated items within a single filter have an AND between them. If
+multiple such filters are passed, they have an OR between those item groups.