Remove deprecated PrefixedLogger interface (#4705)

* Add some tests for `logger`

* Remove deprecated `PrefixedLogger` interface

`PrefixedLogger` has been deprecated for some time, so let's remove it now,
while we have a major version bump.

We can tidy up some of the other logic while we're here.

Unfortunately lots of the code still uses `logger.log` which isn't exposed by
the `Logger` interface, so we need to keep exposing that where it was before.

spec/unit/logger.spec.ts

@@ -0,0 +1,51 @@
+/*
+Copyright 2025 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/* eslint-disable no-console */
+
+import loglevel from "loglevel";
+
+import { logger } from "../../src/logger.ts";
+
+afterEach(() => {
+    jest.restoreAllMocks();
+});
+
+describe("logger", () => {
+    it("should log to console by default", () => {
+        jest.spyOn(console, "debug").mockReturnValue(undefined);
+        logger.debug("test1");
+        logger.log("test2");
+
+        expect(console.debug).toHaveBeenCalledWith("test1");
+        expect(console.debug).toHaveBeenCalledWith("test2");
+    });
+
+    it("should allow creation of child loggers which add a prefix", () => {
+        jest.spyOn(loglevel, "getLogger");
+        jest.spyOn(console, "debug").mockReturnValue(undefined);
+
+        const childLogger = logger.getChild("[prefix1]");
+        expect(loglevel.getLogger).toHaveBeenCalledWith("matrix-[prefix1]");
+        childLogger.debug("test1");
+        expect(console.debug).toHaveBeenCalledWith("[prefix1]", "test1");
+
+        const grandchildLogger = childLogger.getChild("[prefix2]");
+        expect(loglevel.getLogger).toHaveBeenCalledWith("matrix-[prefix1][prefix2]");
+        grandchildLogger.debug("test2");
+        expect(console.debug).toHaveBeenCalledWith("[prefix1][prefix2]", "test2");
+    });
+});