LICENSE: event_hdl: fix GPL license version

As spotted by user @songliumeng in GH #2463, there was a mixup between
LGPL and GPL in event_hdl source file: GPL version '2.1' was referenced
instead of '2'. Clarify that this is indeed GPL by making use of the
banner provided in doc/gpl.txt.

This should be backported in 2.8 with 68e692d ("MINOR: event_hdl: add
event handler base api")

(cherry picked from commit e9261fff2925aa3f489d82e6fae73d57cfbcdbc4)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 6025f5f488566c6f087cf6a9b17fd6acf6e69bf2)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/event_hdl.c b/src/event_hdl.c
index 7641486..89f235b 100644
--- a/src/event_hdl.c
+++ b/src/event_hdl.c
@@ -3,11 +3,19 @@
  *
  * Copyright 2022 HAProxy Technologies
  *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License
- * as published by the Free Software Foundation; either version
- * 2.1 of the License, or (at your option) any later version.
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
  *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
  */
 
 #include <string.h>