[#167] stack2cabal with lts 15.7

Description

Problem: Currently stack2cabal is built with old lts-14.15, which forces developers to store two snapshots. Also stack2cabal cannot be built with new lts due to incompatibility with the last HsYAML library.

Solution: Pin newest HsYAML version that is usable with stack2cabal.

Related issue(s)

Resolves #167 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

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