revert(api): remove /plugins/schema/:name (#11313)

Partially reverts #10846

Even though we have been emitting deprecation warnings when the endpoint
was used, we didn't remove it in 3.0, and doing it now can be a surprise
breaking change for someone. In fact, someone had to update a project
which was relying on that path, that is linked in #10846

This reverts the removal and replaces the deprecation version with 4.0
instead of 3.0. This PR does not revert the changes in our tests to use
the new endpoint instead of the deprecated one.

kong/api/routes/plugins.lua

@@ -3,6 +3,7 @@ local utils = require "kong.tools.utils"
 local reports = require "kong.reports"
 local endpoints = require "kong.api.endpoints"
 local arguments = require "kong.api.arguments"
+local api_helpers = require "kong.api.api_helpers"
 
 
 local ngx = ngx
@@ -119,6 +120,23 @@ return {
     PATCH = patch_plugin
   },
 
+  ["/plugins/schema/:name"] = {
+    GET = function(self, db)
+      kong.log.deprecation("/plugins/schema/:name endpoint is deprecated, ",
+                           "please use /schemas/plugins/:name instead", {
+                             after = "1.2.0",
+                             removal = "4.0.0",
+                           })
+      local subschema = db.plugins.schema.subschemas[self.params.name]
+      if not subschema then
+        return kong.response.exit(404, { message = "No plugin named '" .. self.params.name .. "'" })
+      end
+
+      local copy = api_helpers.schema_to_jsonable(subschema.fields.config)
+      return kong.response.exit(200, copy)
+    end
+  },
+
   ["/plugins/enabled"] = {
     GET = function()
       local enabled_plugins = setmetatable({}, cjson.array_mt)