chore: modernize to minimum_otp_vsn 27

[kivra-kenand]

Modernize build, deps and CI

Requires OTP 27+ — JSON now goes through the stdlib json module.

• Drops jsx in favour of stdlib json. Encode/decode output was diffed against jsx across map and proplist mode before the swap; behaviour is unchanged for all valid input, including proplist objects, jsx's [{}] empty object, and duplicate-key ordering.
• Drops mochiweb test dep — dead weight. The vendored test/mochiweb_util.erl oracle shadowed the dep's module of the same name, and the dep was only reachable via a guess_mime/1 no test calls.
• Bumps hackney 4.5.2 → 4.7.4, erlsom 1.5.1 → 1.5.2, meck 0.8.13 → 1.2.0, proper → 1.5.0 (was unpinned).
• CI now runs build/test and dialyzer across OTP 27, 28 and 29.
• Removes the erlang.mk / rebar2 path (erlang.mk, rebar.config.script, restc.d) — it carried a duplicate dep list that had already drifted (hackney pinned at 1.20.1 there vs 4.5.2 in rebar.config).
• Bumps elvis 4.1.1 → 6.0.0; 4.1.1 can't build on OTP 29 (its lager/goldrush chain uses deprecated catch with warnings_as_errors). elvis.config migrated to the 5.x/6.x format, including five renamed rules that the new elvis otherwise skips silently.

Two behaviour changes to check before upgrading

1. Malformed JSON raises error:unexpected_end / error:{invalid_byte, N} instead of error:badarg. Same error class, so catch error:_ handlers are unaffected — only code matching badarg specifically.
2. {return_maps, true} (tuple form) now returns maps from JSON bodies; it previously returned a proplist. This was a genuine inconsistency — the JSON branch used lists:member/2 while the empty-body and urlencoded branches used proplists:get_bool/2, so the same option gave two different answers. Bare [return_maps] is unaffected.

Commits are split so each one builds and tests green independently.

Worth a grep for badarg and {return_maps, true} across the consuming services before tagging.

[Copilot]

Pull request overview

Modernizes restclient for OTP 27+, replacing JSX with stdlib JSON and simplifying build tooling.

Changes:

• Migrates JSON encoding/decoding to OTP’s json module.
• Updates dependencies, Elvis, tests, and CI for OTP 27–29.
• Removes legacy erlang.mk/rebar2 support and unused dependencies.

Reviewed changes

Copilot reviewed 12 out of 16 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
.gitattributes	Removes erlang.mk diff handling.
.github/workflows/erlang.yml	Tests OTP 27–29.
.gitignore	Removes obsolete build ignores.
Makefile	Retains Elvis tooling only.
README.md	Documents OTP requirement and installation.
elvis.config	Migrates to Elvis 6 rules.
erlang.mk	Removes legacy build system.
rebar.config	Sets OTP minimum and updates dependencies.
rebar.config.script	Removes rebar2 compatibility.
rebar.lock	Refreshes dependency locks.
restc.d	Removes obsolete make metadata.
src/restc.app.src	Removes JSX application dependency.
src/restc.erl	Introduces the public JSON term type.
src/restc_body.erl	Implements stdlib JSON handling.
test/mochiweb_util.erl	Removes unused MIME functionality.
test/restc_SUITE.erl	Updates JSON integration tests.

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