build: update toolchain to Go 1.25.3

[tgross]

We recently updated to Go 1.25.2 but upstream just released a fix for TLS certificate validation that the security patch introduced. Let's upgrade to 1.25.3 before we ship the 1.11.0 beta.

Ref: #26909
Ref: https://go.dev/doc/devel/release#go1.25.3

Description

Testing & Reproduction steps

Links

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad website documentation to reflect this. Refer to
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Reviewer Checklist

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  within the public repository.

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

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