package: add validation step on MRs

General MR

Summary

Adds in a validation step on MR creation events for packages that verifies that Upgrade Notices are included. These notices will later be automatically scraped during releases to speed up that process.

Valid options are:

• None
• N/A
• literally anything

Invalid options are:

• (Include any relevant notes about upgrades here or write "N/A" if there are none) (default template line)
• Nothing

Also errors if the MR is missing an Upgrade Notices section entirely.

Relevant logs/screenshots

Testing using this mr and this pipeline validation branch.

Valid Options:

./test.sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  4064  100  4064    0     0   9801      0 --:--:-- --:--:-- --:--:--  9912
Upgrade Notices: N/A

./test.sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  4065  100  4065    0     0   9654      0 --:--:-- --:--:-- --:--:--  9748
Upgrade Notices: None

 ./test.sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  4075  100  4075    0     0   7808      0 --:--:-- --:--:-- --:--:--  7882
Upgrade Notices: Do x
Do y

Invalid options

./test.sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  4027  100  4027    0     0   9705      0 --:--:-- --:--:-- --:--:--  9821
No Upgrade Notices section found in the description.
Please include an ## Upgrade Notices section and list upgrade notes beneath it.

./test.sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  4049  100  4049    0     0   9732      0 --:--:-- --:--:-- --:--:--  9803
Invalid value for Upgrade Notices: 

./test.sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  4134  100  4134    0     0   9656      0 --:--:-- --:--:-- --:--:--  9750
Upgrade notices were not filled out. Please include upgrade notices or write "N/A" if there are none.

Closes big-bang/bigbang#1694 (closed)