chore: upstream dfinity/motoko-core moved to v2.6.1

[pr-automation-bot-public]

dfinity/motoko-core has moved past the ref the docs are verified against.

Pinned in .sources/upstream.json	v2.4.0
Latest release	v2.6.1
Compare	caffeinelabs/motoko-core@v2.4.0...v2.6.1
Published reference	https://mops.one/core/docs

What to re-check

Every mo:core import and API signature in Motoko code blocks.

Changelog.md

14 line(s) added, 0 removed.

New sections:

• ## 2.6.1
• ## 2.6.0
• ## 2.5.0

Full diff:

diff <(curl -sL https://raw.githubusercontent.com/dfinity/motoko-core/v2.4.0/Changelog.md) \
     <(curl -sL https://raw.githubusercontent.com/dfinity/motoko-core/v2.6.1/Changelog.md)

How to close this

1. Review the changes above against the pages named under "What to re-check".
2. Update those pages if anything they state has changed.
3. Set pinned to v2.6.1 in .sources/upstream.json in the same PR.

If nothing in the docs is affected, bump the pin on its own and say so in the PR.

Procedure: .agents/upstream-tracking.md

[marc0olo]

Done in #370, and this was the one with real content impact. v2.6.0 deprecated every Module.fromX with a toX counterpart, and our examples used five of them across 20 call sites: Nat64.fromNat, Nat8.fromNat, Blob.fromArray, Float.fromInt, Text.fromChar. Each was replaced with the function named in upstream's own @deprecated marker, plus four imports added where a replacement needed a module the block did not import. Pin moved to v2.6.1.