[MCR] Set MCR migration stage to write-both/read-new.

[MCR] Set MCR migration stage to write-both/read-new.

This patch exists to see if CI passes with
$wgMultiContentRevisionSchemaMigrationStage set to
SCHEMA_COMPAT_WRITE_BOTH | SCHEMA_COMPAT_READ_NEW.

NOTE: verify that $wgMultiContentRevisionSchemaMigrationStage
is explicitly set toSCHEMA_COMPAT_WRITE_BOTH | SCHEMA_COMPAT_READ_OLD
in production config (T197816) before merging this.

Bug: T198561
Depends-On: Ib718868d2c768b6ea851355eef047bc0e6593495
Change-Id: I15989adae2b5916577d164c50d7da88774e49324