general: Use deducation guides for std::lock_guard and std::unique_lock

Since C++17, the introduction of deduction guides for locking facilities
means that we no longer need to hardcode the mutex type into the locks
themselves, making it easier to switch mutex types, should it ever be
necessary in the future.

src/web_service/web_backend.cpp

@@ -24,7 +24,7 @@ constexpr u32 TIMEOUT_SECONDS = 30;
 struct Client::Impl {
     Impl(std::string host, std::string username, std::string token)
         : host{std::move(host)}, username{std::move(username)}, token{std::move(token)} {
-        std::lock_guard<std::mutex> lock(jwt_cache.mutex);
+        std::lock_guard lock{jwt_cache.mutex};
         if (this->username == jwt_cache.username && this->token == jwt_cache.token) {
             jwt = jwt_cache.jwt;
         }
@@ -151,7 +151,7 @@ struct Client::Impl {
         if (result.result_code != Common::WebResult::Code::Success) {
             LOG_ERROR(WebService, "UpdateJWT failed");
         } else {
-            std::lock_guard<std::mutex> lock(jwt_cache.mutex);
+            std::lock_guard lock{jwt_cache.mutex};
             jwt_cache.username = username;
             jwt_cache.token = token;
             jwt_cache.jwt = jwt = result.returned_data;