Allow RSA 2047 bit keys (#20272)

Unfortunately it appears that 2048 bit RSA keys can occasionally be created in such
a way that they appear to have 2047 bit length. This PR simply changes our defaults to
allow these.

Fix #20249

Signed-off-by: Andrew Thornton <art27@cantab.net>

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -621,7 +621,7 @@ Define allowed algorithms and their minimum key length (use -1 to disable a type
 
 - `ED25519`: **256**
 - `ECDSA`: **256**
-- `RSA`: **2048**
+- `RSA`: **2047**: We set 2047 here because an otherwise valid 2048 RSA key can be reported as 2047 length.
 - `DSA`: **-1**: DSA is now disabled by default. Set to **1024** to re-enable but ensure you may need to reconfigure your SSHD provider
 
 ## Webhook (`webhook`)