Unconditionally flush session on all statement executions

Unconditionally flush session on all statement executions

This change simplifies session execution logic to always run autoflush
for all statement executions, regardless of whether they are ORM or Core
statements. Previously, autoflush was conditionally applied only when
ORM-related statements were detected, which had become difficult to define
clearly with the unified v2 syntax that blurs the lines between Core and
ORM execution patterns.

The implementation adds a call to self._autoflush() in the Core execution
path (when compile_state_cls is None) in Session._execute_internal(),
ensuring consistent autoflush behavior across all types of SQL execution.

This provides more predictable session behavior and eliminates the
confusion around which statements trigger autoflush.

Also included is some improvement to the migration_21 document

Fixes: #9809
Change-Id: I0b1974f1981c8a747a0feaaca74ed45106b2b160