build+docs: support CentOS Stream 9, drop CentOS 8

Refs: #5181
Change-Id: I2d8bc595bad047ceaa78bd3729848491b843e83d
diff --git a/docs/INSTALL.rst b/docs/INSTALL.rst
index a73b211..789ecc6 100644
--- a/docs/INSTALL.rst
+++ b/docs/INSTALL.rst
@@ -10,9 +10,9 @@
 -  Ubuntu 18.04 (amd64, armhf, i386)
 -  Ubuntu 20.04 (amd64)
 -  Ubuntu 21.10 (amd64)
+-  CentOS Stream 9
 -  macOS 10.15
 -  macOS 11 (Intel only)
--  CentOS 8
 
 NFD is known to work on the following platforms, although they are not officially
 supported:
@@ -110,7 +110,6 @@
 
   .. code-block:: sh
 
-    sudo dnf config-manager --enable powertools  # on CentOS only
     sudo dnf install libpcap-devel systemd-devel
 
 Build
@@ -120,7 +119,7 @@
 
 .. code-block:: sh
 
-    ./waf configure  # on CentOS, add --without-pch
+    ./waf configure
     ./waf
     sudo ./waf install