Summary:
Packed row v2 was a preview feature gated behind the ysql_use_packed_row_v2
flag (DEFINE_RUNTIME_PREVIEW_bool, default false). This converts it to a
kExternal AutoFlag with initial value false and target value true, so packed
row v2 becomes the default. It only takes effect when row packing is enabled.

As a kExternal AutoFlag it activates only after every process in the universe
(including CDC and xCluster consumers) has been upgraded to a version that
understands the v2 format, keeping upgrade and rollback safe.

Also drops the now-invalid --allowed_preview_flags_csv=ysql_use_packed_row_v2
from PgAlterTableTest, since the flag is no longer a preview flag (the explicit
--ysql_use_packed_row_v2 setter in that test is retained).

Addresses #25366 and #22558.

Test Plan: Jenkins

Reviewers: hsunder

Reviewed By: hsunder

Subscribers: ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D53962