Require sebastian/diff in issue-bot instead of relying on PHPUnit to install it

PostGenerator compares the diff it builds with the diff in earlier bot
comments to avoid posting the same comment again. sebastian/diff 9
removed UnifiedDiffOutputBuilder and switched to a different diff
algorithm, so it would produce different diffs for the same errors
and the bot would comment again on issues it already commented on.
Pin the major version the bot has been posting with.

Amp-Thread-ID: https://ampcode.com/threads/T-01a176c6-528a-7430-b296-e66930d153b4

Author: ondrejmirtes

issue-bot/composer.json

@@ -13,6 +13,7 @@
 		"react/child-process": "^0.6.7",
 		"react/event-loop": "^1.6",
 		"react/promise": "^3.3",
+		"sebastian/diff": "^7.0",
 		"symfony/console": "^8.1",
 		"symfony/finder": "^8.1"
 	},