Fix worksheetTitle values in set-requires-testing-status.yml

[nlon982]

1. Fix update_worksheet's worksheetTitle variable

   This is a random guess at what caused the issue Hugh described:

   read_worksheet was successful
   update_worksheet used the wrong worksheetTitle (it uses the first worksheet, rather than "Dev QA")

2. Change to make read_worksheet's worksheetTitle not hardcoded.

---

Testing to make sure nothing has been broken:
Merge in a content-hub PR and check its status in "Dev QA" is updated to "REQUIRES TESTING"

[nlon982]

Note, one place this is called is content-hub. I used this when considering the changes:
https://github.com/SpalkLtd/content-hub/blob/master/.github/workflows/set-req-testing-status.yml