Conda: remove JLAP and rely on upstream sharded repodata if available

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conda-forge is ready for CEP 16 now, and JLAP support was dropped in py-rattler >0.22.0.
However, none of the Chinese mirrors listed in our default configuration has been synchronized with that, so we must wait until the JLAP endpoints are scheduled to be removed from conda-forge.
Local-generated sharded repodata will remain available for channels without CEP 16 support.

[hingebase]

See also conda/rattler#2093

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JLAP support was dropped from conda-forge on approximately June 18, 2026.

At this moment, only conda.anaconda.org and prefix.dev provide sharded repodata, and they are incompatible with each other. When a client requests sharded repodata, the conda.anaconda.org one should always be returned. When we require sharded repodata ourselves (for SHA-256 validation), we can request any one of them, and fall back to the other. Use conda.anaconda.org by default; users can switch to prefix.dev by editing [upstream.conda.channel-alias].