[meta] Drop implicit autocommit on Transaction destruction

Problem/Motivation

As described in numerous issues (see related), it is desirable to drop the auto-commit of database transactions during the destruction of Drupal's Transaction objects.

This meta issue is to track the overall plan to achieve that.

Proposed resolution

Done:

1. Start allowing explicit COMMIT/RELEASE SAVEPOINT from the Transaction objects: #3398767: Allow returning explicitly to the prior nesting level in transactions (aka allow explicit COMMIT in Transaction objects)
2. Convert core to always explicitly commit/release:
   • #3406985: Convert all transactions in core to use explicit ::commitOrRelease()
   • #3583849: Deprecate PgSql Connection::*Savepoint() methods

To do:

1. Deprecate auto-commit on Transaction destruction: #3584238: Deprecate implicit commit-on-destruct
2. #3584465: Fix inconsistencies in the TransactionManager behavior
3. Ensure the event dispatcher is available before the container is ready: #3492391: Make the event dispatcher available before container full bootstrap
4. #3348590: Add transaction-related events to the Database API
5. Move post transaction callback execution to an event-based process and deprecate execution on Transaction destruction: issue tbd
6. #3584322: Cleanup transactions in try/catch blocks
7. Clean up deprecations (Drupal 13 or 14)

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet