libavrocpp: use archive.apache.org for 1.12.1 sources

[michalkas]

Summary

Changes to recipe: libavrocpp/1.12.1

Motivation

libavrocpp/1.12.1 can no longer be installed at all: its source archive 404s.

dlcdn.apache.org is Apache's CDN for current releases only — once a version is
superseded, the tarball is removed. 1.12.1 has been superseded by 1.12.2, so the
URL in conandata.yml is now dead:

$ curl -sI https://dlcdn.apache.org/avro/avro-1.12.1/avro-src-1.12.1.tar.gz
HTTP/2 404

$ curl -sI https://archive.apache.org/dist/avro/avro-1.12.1/avro-src-1.12.1.tar.gz
HTTP/1.1 200 OK
Content-Length: 4099115

fixes #30783
Fix bug as reported in #30783 (comment)

Details

Repointed the 1.12.1 source URL from dlcdn.apache.org to archive.apache.org,
which permanently retains all past Apache releases. The artifact is byte-identical
— sha256 is unchanged, which the build verifies.

archive.apache.org is already the established convention for Apache sources.

Not a duplicate of #30781, but related. That PR replaces 1.12.1 with 1.12.2.
Worth flagging for reviewers: bumping the version does not by itself fix the
build, because lang/c++/include/avro/Exception.hh is identical in
release-1.12.1 and release-1.12.2 — both call fmt::format while including
only <fmt/core.h>. Since fmt 12.2.0, fmt/core.h no longer declares
fmt::format, so both versions fail to compile against it. (Upstream main has
since dropped fmt for std::format, so there is no 1.12.x release with a fix.)

Known CI failure, disclosed up front. fmt/[>=12 <13] resolves to
fmt/12.2.0, and the compile then fails in Exception.hh regardless of where the
sources are downloaded from — i.e. this is pre-existing and orthogonal to the URL
change. Measured locally with Conan 2.27.0, macOS armv8, apple-clang 21, varying
only the fmt version:

fmt resolved	Result
12.1.0	passes (builds + test_package runs)
12.2.0	fails: 16× no member named 'format' in namespace 'fmt' at Exception.hh:37

Two options to make CI green, happy to do either here or in a follow-up —
whichever the team prefers:

1. A one-line source fix, #include <fmt/core.h> → #include <fmt/format.h>
   (verified: all four configurations below pass against fmt 12.2.0 with it).
   Precedent for this shape exists, e.g. rmm adds a missing <array> include
   via replace_in_file.
2. Cap the requirement to fmt/[>=12 <12.2] — no source patching, but it blocks
   consumers who need fmt 12.2.

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• Read the contributing guidelines
• Checked that this PR is not a duplicate: list of PRs by recipe
• If this is a bug fix, please link related issue or provide bug details
• Tested locally with at least one configuration using a recent version of Conan

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