Replace all calls to db_transaction, which is deprecated

Only 1 instance of "db_transaction()" found. #2 Looks good.

Thanks.

Read the issue summary carefully about test usage replacements.

1. +++ b/core/lib/Drupal/Core/Database/database.api.php
   @@ -150,7 +150,7 @@
   + * call @code $txn = \Drupal::database()->startTransaction(); @endcode The transaction will
   

   Documentation will be updated in separate ticket.

2. +++ b/core/lib/Drupal/Core/Database/database.api.php
   @@ -161,7 +161,7 @@
   + *   $txn = \Drupal::database()->startTransaction();
   

   Documentation will be updated in separate ticket.

3. +++ b/core/lib/Drupal/Core/EventSubscriber/MenuRouterRebuildSubscriber.php
   @@ -27,16 +29,35 @@ class MenuRouterRebuildSubscriber implements EventSubscriberInterface {
   +  public function __construct(LockBackendInterface $lock, MenuLinkManagerInterface $menu_link_manager, Connection $database) {
   

   Not sure, this change is required or not in the context of ticket.

@Sharique

point 1 and 2: Done
Point 3: I think we need to inject the dependency.

+++ b/core/lib/Drupal/Core/EventSubscriber/MenuRouterRebuildSubscriber.php
@@ -27,16 +29,35 @@ class MenuRouterRebuildSubscriber implements EventSubscriberInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('database')
+    );
   }

Not correct create() method implementation. Database service should pass through the third argument. Anyway, I guess it is redundant code. Database service injecting already defined in core.services.yml

+++ b/core/lib/Drupal/Core/EventSubscriber/MenuRouterRebuildSubscriber.php
@@ -27,16 +29,35 @@ class MenuRouterRebuildSubscriber implements EventSubscriberInterface {
+  protected $database;
...
+  public function __construct(LockBackendInterface $lock, MenuLinkManagerInterface $menu_link_manager, Connection $database) {
...
+    $this->database = $database;

@@ -55,7 +76,7 @@ public function onRouterRebuild(Event $event) {
+      $transaction = $this->database->startTransaction();

grep through core shows that *this->connection* is mostly used against *this->database*

Rerolled, added triggering deprecated error & test
Also converted core/tests/Drupal/KernelTests/Core/Database/TransactionTest.php
I'm sure changes are in scope cos test for inTransaction() should be made from the same connection as startTransaction() requested

Linked to #2947775: Move setting default target out of db_merge() and other deprecated db_* functions cos setting defaults needs solved somehow

There's is only 2 mentions in docs but that's separate issue

$ git grep -F 'this->database'|wc -l
430
$ git grep -F 'this->connection'|wc -l
544

Transactions are per connection but database mostly used in backend specific implementation

Rerolled, added reference to CR in the deprecation msg, adjusted test to benefit from the $connection property being available from DatabaseTestBase, changed in test the returns to markTestSkipped where necessary.

If it turns back green this should be close to ready now.

Looks good.

Committed/pushed to 8.7.x, thanks!