Backport of [NET-4968] Upgrade Go to 1.21 into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #20062 to be assessed for backporting due to the inclusion of the label backport/1.15.

🚨

Warning automatic cherry-pick of commits failed. If the first commit failed,
you will see a blank no-op commit below. If at least one commit succeeded, you
will see the cherry-picked commits up to, not including, the commit where
the merge conflict occurred.

The person who merged in the original PR is:
@zalimeni
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: POST https://api.github.com/repos/hashicorp/consul/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

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Description

Upgrade Go to 1.21.

This is needed in advance of the EOL of Go 1.20 ~Feb. 1. The ideal timing is before our next patch release scheduled for the end of January.

A follow-up change will introduce toolchain directives to pin to the latest version; the hope is to use a root-level go.work to accomplish this, which requires some additional changes to support. See #20058 for more context.

That PR should set this change up to cleanly backport and sync without intervention 🤞🏻

Testing & Reproduction steps

Tests continue to pass, and Go 1.21 can be observed as the output of get-go-version.

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

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Overview of commits

• 01ca961 - e71ae3a

[hashicorp-cla]

All committers have signed the CLA.

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR

[zalimeni]

Resolved whitespace and Makefile no-op conflicts.