process: Post-History ↦ Discussion

Also line up with additional items in the lines below.

```
sed -i -z 's/  Post-History: /  Discussion:   /' bip-0*.md
sed -i -z 's/  Post-History: /  Discussion:   /' bip-0*.mediawiki
```

bip-0347.mediawiki

@@ -9,7 +9,7 @@
   Type: Specification
   Created: 2023-12-11
   License: BSD-3-Clause
-  Post-History: 2023-10-21: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2023-October/022049.html [bitcoin-dev] Proposed BIP for OP_CAT
+  Discussion: 2023-10-21: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2023-October/022049.html [bitcoin-dev] Proposed BIP for OP_CAT
 </pre>
 
 ==Abstract==