[BUGFIX] Change EM composer mode message to info level

As https://review.typo3.org/#/c/44025/ enabled extensionmanager
offline mode for composer installations, the composer mode message
should no longer be a warning but an info.

Releases: master, 7.6
Fixes: #79107
Change-Id: Ic9fcb134c113172de504833ed81f7a46a0ba4e2c
Reviewed-on: https://review.typo3.org/51059


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>
Tested-by: Philipp Gampe <philipp.gampe@typo3.org>