docs: remove dead URLs

- KNOWN_BUGS: remove dead URL
- ECH: remove two dead URLs
- MAIL-ETIQUETTE: remove dead URL

docs/ECH.md

@@ -76,8 +76,6 @@ The above works for these test sites:
 
 ```sh
 https://defo.ie/ech-check.php
-https://draft-13.esni.defo.ie:8413/stats
-https://draft-13.esni.defo.ie:8414/stats
 https://crypto.cloudflare.com/cdn-cgi/trace
 https://tls-ech.dev
 ```

docs/KNOWN_BUGS

@@ -311,7 +311,7 @@ problems may have been fixed or changed somewhat since this was written.
 
  libcurl fails to build with MIT Kerberos for Windows (KfW) due to KfW's
  library header files exporting symbols/macros that should be kept private to
- the KfW library. See ticket #5601 at https://krbdev.mit.edu/rt/
+ the KfW library.
 
 6.3 NTLM in system context uses wrong name
 

docs/MAIL-ETIQUETTE.md

@@ -223,8 +223,7 @@ mails to your friends. We speak plain text mails.
 ### Quoting
 
 Quote as little as possible. Just enough to provide the context you cannot
-eave out. A lengthy description can be found
-[here](https://www.netmeister.org/news/learn2quote.html).
+leave out.
 
 ### Digest