update end-of-life.txt

[Bruno Haible]

Just realized that in our end-of-life database (in maint-tools/),
for RHEL we should consider the end of "maintenance support", not the end of
"extended life phase". The reason is that during the "extended life phase"
the RHEL customers receive no bug fixes, no security fixes, no installation
support. So, it seems that during this phase
  - all they receive from the Red Hat support is advice and doc pointers,
  - most customers have already left the OS (because who wants to rely on
    an OS that does not receive security updates?).

As a consequence, we can consider RHEL 6 already dead, and RHEL 7 dead in
4 months.

[1] https://access.redhat.com/support/policy/updates/errata


diff --git a/end-of-life.txt b/end-of-life.txt
index 2a511e2..ab966af 100644
--- a/end-of-life.txt
+++ b/end-of-life.txt
@@ -9,10 +9,11 @@ End-of-life / end-of-support data for various OS releases
   Reference: https://endoflife.date/debian
 
 * RHEL / CentOS
-  RHEL / CentOS 5:               2020-11-30 †
-  RHEL / CentOS 6:               2024-06-30
-  RHEL / CentOS 7:               2026-06-30
-  RHEL / CentOS 8 / AlmaLinux 8: 2029-03-01
+  For RHEL, consider the end of "maintenance support", not the end of 
"extended life phase".
+  RHEL / CentOS 5:               2017-03-31 †
+  RHEL / CentOS 6:               2020-11-30 †
+  RHEL / CentOS 7:               2024-06-30
+  RHEL / CentOS 8 / AlmaLinux 8: 2029-05-31
   RHEL / AlmaLinux 9:            2032-05-31
   Reference: 
https://en.wikipedia.org/wiki/Red_Hat_Enterprise_Linux#Version_history_and_timeline
              https://access.redhat.com/support/policy/updates/errata