[main] Upgrade the Golang version to go1.25.6

[vitess-bot]

This Pull Request bumps the Golang version to go1.25.6 and the bootstrap version to 50.

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

There are a few manual steps remaining:

• Make sure you update the Golang version used in the previous and next release branches for the Upgrade/Downgrade tests.
• Build and Push the bootstrap images to Docker Hub, the bot cannot handle that.

cc @vitessio/release

[vitess-bot]

Hello! 👋

This Pull Request is now handled by arewefastyet. The current HEAD and future commits will be benchmarked.

You can find the performance comparison on the arewefastyet website.

[vitess-bot]

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[timvaillancourt]

Seems this auto update is changing 3 -> 6 but the SHA is the same. That sounds wrong

Is this expected @frouioui?

[mhamza15]

This is due to us pinning our dependencies/images by SHA recently: #19143. I'll see if I can fix up the automation to update the SHA as well, or alternatively, letting dependabot handle image updates. Only concern with allowing dependabot to do it is that it'll leave behind go.mod, so better off to do it this way most likely.

[mhamza15]

#19178

[mhamza15]

Closing to regenerate with digests using #19178