[TASK] Replace IRC with Slack in composer support section

Remove reference to irc as it is not used anymore. Linking
to Slack instead.

Resolves: #87720
Releases: master, 9.5, 8.7
Change-Id: I7dea0d62329e8e361b8ac37a1d18b9f9f8603943
Reviewed-on: https://review.typo3.org/c/59705


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>