build(deps): update axllent/mailpit to v1.21.8

This MR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [axllent/mailpit](https://github.com/axllent/mailpit) | patch | `v1.21.6` -> `v1.21.8` |

MR created with the help of [el-capitano/tools/renovate-bot](https://gitlab.com/el-capitano/tools/renovate-bot).

**Proposed changes to behavior should be submitted there as MRs.**

---

### Release Notes

<details>
<summary>axllent/mailpit (axllent/mailpit)</summary>

### [`v1.21.8`](https://github.com/axllent/mailpit/blob/HEAD/CHANGELOG.md#v1218)

[Compare Source](axllent/mailpit@v1.21.7...v1.21.8)

##### Chore

-   Update node dependencies
-   Update Go dependencies

##### Fix

-   **db:** Remove unused FOREIGN KEY REFERENCES in message_tags table ([#&#8203;374](axllent/mailpit#374))

### [`v1.21.7`](https://github.com/axllent/mailpit/blob/HEAD/CHANGELOG.md#v1217)

[Compare Source](axllent/mailpit@v1.21.6...v1.21.7)

##### Chore

-   Update node dependencies
-   Update Go dependencies
-   Bump Go version for automated testing
-   Move smtpd & pop3 modules to internal
-   Stricter SMTP 'MAIL FROM' & 'RCPT TO' handling ([#&#8203;409](axllent/mailpit#409))
-   Display "To" details in mobile messages list
-   Display "From" details in message sidebar (desktop) ([#&#8203;403](axllent/mailpit#403))

##### Fix

-   Ignore unsupported optional SMTP 'MAIL FROM' parameters ([#&#8203;407](axllent/mailpit#407))
-   Prevent splitting multi-byte characters in message snippets ([#&#8203;404](axllent/mailpit#404))

##### Testing

-   Add smtpd tests

</details>

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### Configuration

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---

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chezmoi/dot_config/aquaproj-aqua/aqua.yaml

@@ -268,7 +268,7 @@ packages:
 - name: yassinebridi/serpl@0.3.3
   description: A simple terminal UI for search and replace, ala VS Code
   link: https://github.com/yassinebridi/serpl
-- name: axllent/mailpit@v1.21.6
+- name: axllent/mailpit@v1.21.8
   description: An email and SMTP testing tool with API for developers
   link: https://github.com/axllent/mailpit
 - name: mrjackwills/oxker@v0.9.0