[BUGFIX] Accessing unavailable site config triggers notice instead of warning

Accessing a not available configuration via TypoScript can be
intentional (e.g. by using it in an if check).
Therefore the log entry severity should rather be downgraded to a notice.

Releases: 10.4, 11.5, main
Resolves: #99465
Change-Id: I26ed4e96290ce6839c32cc60a7cbcf7fa24d0f2b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77289


Tested-by: Helmut Hummel <typo3@helhum.io>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Helmut Hummel <typo3@helhum.io>