[BUGFIX] Fix RTE file links migration

Links in RTE might be stored as <LINK or <link.
Currently the upgrade wizards lacks support for <LINK.
Add this to the regular expression.

Resolves: #51800
Releases: 6.3, 6.2
Change-Id: Ic22983db1144fb1317d6102907c792871e1eeaed
Reviewed-on: https://review.typo3.org/30820
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters