Placeholders are only replaced once

[ggwpez]

Describe the bug

Having a description that contains ${pull_number} more than once only results it in being replaced once.
Example here: paritytech/polkadot-sdk#5634

You can see the placeholder only being replaced once.

To Reproduce
Use a placeholder more than once.

Expected behavior
Should replace it everywhere.

Additional context

It should probably use replaceAll instead of replace here.