[main] Upgrade the Golang version to go1.26.4

[vitess-bot]

This Pull Request bumps the Golang version to go1.26.4 and the bootstrap version to 57.

Do not trust the bot blindly. A thorough code review must be done to ensure all the files have been correctly modified.

cc @vitessio/release

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[vitess-bot]

Hello! 👋

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[vitess-bot]

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[promptless]

Promptless prepared a documentation update related to this change.

Triggered by PR #20337

The macOS build-from-source guide pinned go1.26.1 in its example Go install commands. Since this PR bumps the main toolchain to go1.26.4, the doc example is now stale — the suggestion updates those commands to go1.26.4.

Review: Update macOS build-from-source Go version example to go1.26.4