build: require gcc >= 9.1, recommend gcc >= 10.2

Similarly, recommend clang 10 or later. On macOS, the minimum Xcode
version is now 11, the recommended one is still 13 or later.

Change-Id: Id4dd3007b62f23e7c5ce57b2764debc5eeab98f1
diff --git a/docs/INSTALL.rst b/docs/INSTALL.rst
index e03d946..84b6784 100644
--- a/docs/INSTALL.rst
+++ b/docs/INSTALL.rst
@@ -7,22 +7,21 @@
 NFD is built against a continuous integration system and has been tested on the
 following platforms:
 
-- Ubuntu 20.04 (focal)
 - Ubuntu 22.04 (jammy)
 - Ubuntu 24.04 (noble)
 - Debian 11 (bullseye)
 - Debian 12 (bookworm)
 - CentOS Stream 9
-- macOS 12 / 13 / 14 / 15
+- macOS 13 / 14 / 15
 
 NFD should also work on the following platforms, although they are not officially
 supported:
 
 - Any other recent version of Ubuntu not listed above
-- Fedora >= 33
-- Alpine >= 3.12
+- Fedora >= 34
+- Alpine >= 3.14
 - Any version of Raspberry Pi OS based on Debian 11 (bullseye) or later
-- macOS >= 10.15
+- macOS >= 11
 - FreeBSD >= 12.2
 
 .. _Install NFD on Ubuntu Linux using the NDN PPA repository: