[BUGFIX] Available extension list not updated on CLI

On installing a manually downloaded extension usign the
extension:install CLI API, the list of available extensions does not
get updated prior to the installation, causing it to fail.

Resolves: #53071
Releases: 6.2
Change-Id: I8610f27b8fe36c15f02d88e7b4275099e5443b96
Reviewed-on: https://review.typo3.org/24972
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
[TASK] EXT:documentation: Remove box around links

Since the base CSS has been updated to underline links on
mouse hover, the box that is shown in EXT:documentation
should be removed.

Resolves: #52935
Releases: 6.2
Change-Id: I385b703dc533ccfecf85aa86537bfed43ec2a995
Reviewed-on: https://review.typo3.org/24888
Reviewed-by: Sascha Egerer
Tested-by: Sascha Egerer
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
[TASK] Add xlf to [SYS][textfile_ext]

Add the xlf file extension the list of textfile extensions in the default
configuration.

Resolves: #53136
Releases: 6.2
Change-Id: I2ea509c8a027bb21d77a59313cce4dfa9495c280