chore(doc): removes references to the former birdsite

Author: sverweij

.github/CONTRIBUTING.md

@@ -14,7 +14,7 @@ To make this as easy as possible for you, here's some simple guidelines:
 - In turn, we try to **respond within two weeks**.  
   This might or might not include an actual code fix.
 - If there's something that doesn't fit an issue, feel free to contact us on
-  twitter [@mcmeadow](https://twitter.com/mcmeadow).
+  Mastodon: [@mcmeadow@mstdn.social](https://mstdn.social/@mcmeadow).
 
 ### Contributing code
 

CODE_OF_CONDUCT.md

@@ -60,7 +60,7 @@ representative at an online or offline event.
 
 Instances of abusive, harassing, or otherwise unacceptable behavior may be
 reported to the community leaders responsible for enforcement by e-mail at
-sander_verweij@yahoo.com or by direct message on twitter: [@mcmeadow](https://twitter.com/mcmeadow).
+sander_verweij@yahoo.com or by direct message on mastodon: [@mcmeadow@mstdn.social](https://mstdn.social/@mcmeadow)
 All complaints will be reviewed and investigated promptly and fairly.
 
 All community leaders are obligated to respect the privacy and security of the

SECURITY.md

@@ -11,7 +11,6 @@ If you think you have found a vulnerability, please report responsibly. Don't
 create GitHub issues for security issues. Instead, please send an email to
 sander_verweij@yahoo.com and/ or a direct message (DM) to
 [@mcmeadow@mstdn.social](https://mstdn.social/@mcmeadow) on Mastodon
-(or on [twitter](https://twitter.com/mcmeadow) while it lasts)
 and we'll look into it as soon as we can.
 
 We appreciate any responsible disclosure of vulnerabilities that might impact