Fix Garden Linux Bumper

[Molter73]

Description

Garden Linux changed how it names its GCP images, so we cannot get that
from the assets of a release directly. Instead, we need to parse the
markdown text on the release itself and get it from there.

Checklist

• Investigated and inspected CI test results
• Updated documentation accordingly

Automated testing

• Added unit tests
• Added integration tests
• Added regression tests

If any of these don't apply, please comment below.

Testing Performed

• Run the bumper from this PR and it generated another PR with the correct image name: Update Garden Linux VM #2104

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.40%. Comparing base (215e5d4) to head (c69c1dd).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2103   +/-   ##
=======================================
  Coverage   28.40%   28.40%           
=======================================
  Files          94       94           
  Lines        5717     5717           
  Branches     2531     2531           
=======================================
  Hits         1624     1624           
  Misses       3383     3383           
  Partials      710      710           

Flag	Coverage Δ
collector-unit-tests	28.40% <ø> (ø)

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