tests: update ruby bundle v1 spread test

[mr-cal]

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Ruby's bundler v3.4.0 dropped support for Ruby < v2.6, which broke one of our spread tests for the core18 / v1 Ruby plugin.

[mr-cal]

I'm cautious to make changes to the v1 plugin itself, due to the fact that it's been stable for a long time and that I don't know much about Ruby.

I am considering updating the documentation to mention the BUNDLE_GEMFILE environment variable. Or it could become a parameter in the snapcraft.yaml (i.e. ruby-gemfile-location: <path-to-gemfile>).

Aside: Ruby 2.6 is EOL and the latest supported release is 3.2. Not sure if/when it's appropriate to update the default Ruby version.

[lengau]

For now I think this is a good minimal change to get this spread test fixed. However, if it works on Ruby 2.7 that'd probably be reasonable since it should be fully backwards compatible.

In theory Ruby 3 is backwards compatible too, but we should probably make a separate project for bringing our ruby deps up to the latest.

[codecov-commenter]

Codecov Report

Merging #4013 (6182e8d) into main (2afca58) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4013   +/-   ##
=======================================
  Coverage   91.61%   91.61%           
=======================================
  Files          77       77           
  Lines        5153     5153           
=======================================
  Hits         4721     4721           
  Misses        432      432           

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