[BUGFIX] Detect unix-styled absolute paths on Windows systems

Currently PathUtility::isAbsolutePath and GeneralUtility::isAbsPath
only the for ":\" to detect absolute paths on Windows systems. This
patch provides even ":/" as unix-styled paths are mostly used now.

Resolves: #52773
Releases: 6.2, 6.1, 6.0
Change-Id: I1e132bbe74394861af4bf02a22287c9496953150
Reviewed-on: https://review.typo3.org/24724
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes