test: update Pipeline LTS matrix

[waveywaves]

Changes

• Synchronize the Pipeline LTS E2E matrix with the upstream supported-release list.
• Add v1.15.0 and remove v1.3.4.

Submitter Checklist

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.90%. Comparing base (b6fe1cf) to head (913b5dc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1885      +/-   ##
==========================================
- Coverage   61.95%   61.90%   -0.05%     
==========================================
  Files          64       64              
  Lines        4071     4071              
==========================================
- Hits         2522     2520       -2     
- Misses       1268     1269       +1     
- Partials      281      282       +1     

Flag	Coverage Δ
unit-tests	61.90% <ø> (-0.05%)	⬇️

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[infernus01]

/lgtm

[infernus01]

Closing this in favour of @khrm 's PR - #1884