process: Author ↦ Authors

```
sed -z -i 's/Author: /Authors: /' bip-0*.md
sed -z -i 's/Author: /Authors: /' bip-0*.mediawiki
```

Also align correctly in case of multiple authors.

bip-0351.mediawiki

@@ -2,8 +2,8 @@
   BIP: 351
   Layer: Applications
   Title: Private Payments
-  Author: Alfred Hodler <alfred_hodler@protonmail.com>
-          Clark Moody <clark@clarkmoody.com>
+  Authors: Alfred Hodler <alfred_hodler@protonmail.com>
+           Clark Moody <clark@clarkmoody.com>
   Status: Draft
   Type: Informational
   Created: 2022-07-10