[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/59713
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>

composer.json

@@ -20,7 +20,7 @@
 	"support": {
 		"general": "https://typo3.org/support/",
 		"issues": "https://forge.typo3.org",
-		"irc": "irc://irc.freenode.net/#typo3-cms",
+		"slack": "https://typo3.slack.com",
 		"news": "nntp://lists.typo3.org"
 	},
 	"config": {