Alter hook_task TEXT fields to LONGTEXT (#20038) (#20041)

Mysql TEXT has a limit of 64KB, change this to LONGTEXT in mysql only so we can have bigger hook payloads.

Postgresql has unlimited TEXT - https://www.postgresql.org/docs/current/datatype-character.html
Sqlite has unlimited TEXT - https://www.sqlitetutorial.net/sqlite-data-types/#:~:text=The%20maximum%20length%20of%20TEXT,SQLite%20supports%20various%20character%20encodings.

Backport of #20038

Co-authored-by: zeripath <art27@cantab.net>

models/migrations/migrations.go

@@ -389,6 +389,8 @@ var migrations = []Migration{
 	NewMigration("allow to view files in PRs", addReviewViewedFiles),
 	// v216 -> v217
 	NewMigration("Improve Action table indices", improveActionTableIndices),
+	// v217 -> v218
+	NewMigration("Alter hook_task table TEXT fields to LONGTEXT", alterHookTaskTextFieldsToLongText),
 }
 
 // GetCurrentDBVersion returns the current db version