fix(plugins): add realm to removed fields

Basic-auth added new field "realm" but it was not added to "removed_fields" which
breaks backwards compat between new CPs and old DPs.

KAG-4516

(cherry picked from commit 420b3b6)

changelog/unreleased/kong/fix-realm-compat-changes-basic-auth.yml

@@ -0,0 +1,3 @@
+message: "**Basic-Auth**: Fix an issue of realm field not recognized for older kong versions (before 3.6)"
+type: bugfix
+scope: Plugin

kong/clustering/compat/removed_fields.lua

@@ -115,5 +115,8 @@ return {
     opentelemetry = {
       "sampling_rate",
     },
+    basic_auth = {
+      "realm"
+    },
   },
 }

spec/02-integration/09-hybrid_mode/09-config-compat_spec.lua

@@ -468,6 +468,22 @@ describe("CP/DP config compat transformations #" .. strategy, function()
         end)
       end)
     end)
+
+    describe("www-authenticate header in plugins (realm config)", function()
+      it("[basic-auth] removes realm for versions below 3.6", function()
+        local basic_auth = admin.plugins:insert {
+          name = "basic-auth",
+        }
+
+        local expected_basic_auth_prior_36 = utils.cycle_aware_deep_copy(basic_auth)
+        expected_basic_auth_prior_36.config.realm = nil
+
+        do_assert(utils.uuid(), "3.5.0", expected_basic_auth_prior_36)
+
+        -- cleanup
+        admin.plugins:remove({ id = basic_auth.id })
+      end)
+    end)
   end)
 end)