chore: upgrade to Elixir 1.20

[BobbieBarker]

What

Upgrades schema_cache's dev/CI toolchain to Elixir 1.20.1 / OTP 28:

• .tool-versions: 1.19.5-otp-28 to 1.20.1-otp-28
• CI ELIXIR_VERSION: 1.19 to 1.20
• The published-library floor (elixir: "~> 1.14" in mix.exs) is unchanged, so consumers on Elixir 1.14 through 1.20 are unaffected. Only the toolchain we develop and test against moves.

Why the elixir_cache dep changes

Elixir 1.20 ships a new compile-time type checker. It flags the elixir_cache Cache macro: when a cache module passes opts as a compile-time-constant tuple, the macro generates an adapter_options!/1 catch-all clause that is statically unreachable, and 1.20 reports it as a never-used clause. This trips our test-only SchemaCache.Test.ElixirCacheRedis fixture and fails the CI quality job's mix compile --warnings-as-errors.

The fix lives on MikaAK/elixir_cache#47 ("Support Elixir 1.20"), which has not yet been released to hex (latest hex 0.4.8 still trips the warning). So the test-only dep is temporarily pinned to that branch:

{:elixir_cache,
   github: "BobbieBarker/elixir_cache", branch: "chore/elixir-1.20-upgrade", only: :test}

Follow-up: revert to a hex release ({:elixir_cache, "~> 0.x", only: :test}) once #47 merges and ships.

Lock churn

Switching elixir_cache to the git fork re-resolved mix.lock, incidentally bumping unpinned transitive deps within their existing constraints (decimal 2.3.0 to 2.4.1, jason 1.4.4 to 1.4.5, plug 1.19.1 to 1.19.2, telemetry 1.3.0 to 1.4.2) and adding libring (a new transitive dep of the fork's elixir_cache).

Verification (local, Elixir 1.20.1 / OTP 28)

• mix compile --warnings-as-errors (dev and MIX_ENV=test): clean
• mix format --check-formatted: clean
• mix credo --strict: no issues
• mix test: 117 passed (the 2 Redis-adapter files need a live Redis and run in CI, which provides the service)
• mix dialyzer: 0 errors

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.71%. Comparing base (5b70a10) to head (5f92b69).

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
+ Coverage   94.58%   94.71%   +0.13%     
==========================================
  Files           7        7              
  Lines         240      246       +6     
==========================================
+ Hits          227      233       +6     
  Misses         13       13              

see 2 files with indirect coverage changes

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