package/apache: security bump to version 2.4.51

Fixes CVE-2021-42013, for details see [1] and [2].

Change download URL from http://archive.apache.org/dist/httpd to
https://downloads.apache.org/httpd (seems more up to date).

[1] https://downloads.apache.org/httpd/CHANGES_2.4.51
[2] https://httpd.apache.org/security/vulnerabilities_24.html



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>