fix(ThreadChannel): Make fetchStarterMessage() work in forum posts

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
This caused an internal error when used in a forum post. This now fetches the original message in the forum post!

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

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1 Ignored Deployment

Name	Status	Preview	Updated
discord-js	⬜️ Ignored (Inspect)		Sep 19, 2022 at 7:50PM (UTC)

[codecov]

Codecov Report

Merging #8638 (abb3910) into main (c446a84) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head abb3910 differs from pull request most recent head 4fa0055. Consider uploading reports for the commit 4fa0055 to get more accurate results

@@            Coverage Diff             @@
##             main    #8638      +/-   ##
==========================================
+ Coverage   86.66%   86.68%   +0.01%     
==========================================
  Files          86       87       +1     
  Lines        8771     8784      +13     
  Branches     1105     1110       +5     
==========================================
+ Hits         7601     7614      +13     
  Misses       1128     1128              
  Partials       42       42              

Flag	Coverage Δ
builders	100.00% <ø> (ø)
collection	100.00% <ø> (ø)
proxy	74.10% <ø> (ø)
rest	91.99% <ø> (ø)
utilities	100.00% <ø> (?)
voice	63.86% <ø> (ø)
ws	60.47% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/actions/src/formatTag/formatTag.ts	100.00% <0.00%> (ø)

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