[TASK] Migrate core from mysql to mysqli

Resolves: #36419
Releases: 6.1
Change-Id: If34deb026f3118db04477d14b735d76c1606aa97
Reviewed-on: https://review.typo3.org/19280
Reviewed-by: Oliver Klee
Reviewed-by: Felix Kopp
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Steffen Ritter
Tested-by: Philipp Gampe
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack