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-0345.mediawiki

@@ -9,8 +9,8 @@
   Type: Specification
   Created: 2023-02-03
   License: BSD-3-Clause
-  Post-History: 2023-01-09: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2023-January/021318.html [bitcoin-dev] OP_VAULT announcement
-                2023-03-01: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2023-March/021510.html [bitcoin-dev] BIP for OP_VAULT
+  Discussion: 2023-01-09: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2023-January/021318.html [bitcoin-dev] OP_VAULT announcement
+              2023-03-01: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2023-March/021510.html [bitcoin-dev] BIP for OP_VAULT
   Proposed-Replacement: 443
 </pre>