[release-18.0] Upgrade to go1.21.9

[frouioui]

Description

This PR upgrade the go version used to go1.21.9

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

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Backward compatibility

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• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[frouioui]

Still waiting for vitess/bootstrap to finish building, that's why the tests are failing.

[frouioui]

Still waiting for vitess/bootstrap to finish building, that's why the tests are failing.

all good now