Update dependencies and CI matrix, require Elixir 1.15+

[idlehands]

Summary

• Bump all dependencies: Ecto 3.14, Decimal 3.x, ExDoc 0.40, Dialyxir 1.4.8, ExCoveralls 0.18.5, plus transitive deps
• Raise minimum Elixir to 1.15 (Ecto 3.14 requires 1.14+, ExDoc 0.40 requires 1.15+)
• Move CI from the retired ubuntu-20.04 runner to ubuntu-24.04
• New CI matrix: Elixir 1.16.3 through 1.19.6 on OTP 26 through 28, including newest Elixir on oldest supported OTP
• Update checkout/cache actions and switch mix do to the + separator
• Bump version to 1.4.0 with CHANGELOG entry; update README install snippet

Test plan

• mix compile --warnings-as-errors, mix format --check-formatted, mix deps.unlock --check-unused, mix test, mix docs pass locally on Elixir 1.19.5 / OTP 28
• mix test passes locally on Elixir 1.17.3 / OTP 26 and 1.18.4 / OTP 27
• Full five-job CI matrix green on the fork (Sync with upstream, update dependencies and CI matrix idlehands/checker_cab#2)

[coveralls-official]

Coverage Report for CI Build 20

Warning

No base build found for commit 50aaaf4 on main.
Coverage changes can't be calculated without a base build.
If a base build is processing, this comment will update automatically when it completes.

Coverage: 100.0%

Details

• Patch coverage: No coverable lines changed in this PR.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

Requires a base build to compare against. How to fix this →

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Coverage Stats

Relevant Lines:	45
Covered Lines:	45
Line Coverage:	100.0%
Coverage Strength:	16.58 hits per line

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💛 - Coveralls