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-0349.md

@@ -2,8 +2,8 @@
   BIP: 349
   Layer: Consensus (soft fork)
   Title: OP_INTERNALKEY
-  Author: Brandon Black <freedom@reardencode.com>
-          Jeremy Rubin <j@rubin.io>
+  Authors: Brandon Black <freedom@reardencode.com>
+           Jeremy Rubin <j@rubin.io>
   Status: Draft
   Type: Specification
   Created: 2024-11-14