tools/authors: use plaintext instead of markdown for the authors file

- move the authors file to AUTHORS as recommended by https://www.gnu.org/prep/maintain/html_node/Recording-Contributors.html
- simplifies tooling and reduces the risk of invalid markdown injection from a contributor's email address/name

Makefile

@@ -1,5 +1,5 @@
 #!/usr/bin/make -f
 SHELL = /bin/bash
-# update the AUTHORS.md file
+# update the AUTHORS file
 contrib:
-	@mv .github/.mailmap . && printf "|Commits | Author |\n| :---: | --- |\n" > AUTHORS.md && git shortlog -sne | sed -r 's/^\s*([[:digit:]]*?)\s*?(.*?)/|\1|\2|/' >> AUTHORS.md && mv .mailmap .github/.mailmap
+	@mv .github/.mailmap . && printf "Commits|Author\n-------|---------------------------------------------------\n" > AUTHORS && git shortlog -sne >> AUTHORS && mv .mailmap .github/.mailmap

README.md

@@ -2189,7 +2189,7 @@ Contributing guidelines can be found in [.github/CONTRIBUTING.md](.github/CONTRI
 
 ## Authors
 
-The list of authors can be found in [AUTHORS.md](AUTHORS.md).
+The list of authors can be found in the [AUTHORS](AUTHORS) file.
 
 ## License