docs(changelog): version 1.18.20 [citest skip]

Update changelog and .README.html for version 1.18.20

Signed-off-by: Rich Megginson <[email protected]>

Author: richm

CHANGELOG.md

@@ -1,6 +1,14 @@
 Changelog
 =========
 
+[1.18.20] - 2025-08-01
+--------------------
+
+### Other Changes
+
+- tests: Fix verification for LVM pool on top encrypted MD RAID (#549)
+- test: fix error string checking for raid failures (#550)
+
 [1.18.19] - 2025-07-24
 --------------------