use go 1.21.7

[skpratt]

This bumps golang to avoid any change in the BoringCrypto module for FIPS builds: golang/go#65321

[zalimeni]

@skpratt we typically add a changelog entry for these - copying from https://github.com/hashicorp/consul/pull/20062/files#diff-b0b11f4216bda65c2a8dbc878d1e646c7e0355af558118dee097199909b4afd6 you could use:

```release-note:improvement
Upgrade to use Go 1.21.7.
```

[zalimeni]

We should also backport this to n-2 and LTS (1.15) for consistency

[hc-github-team-consul-core]

@skpratt, a backport is missing for this PR [20545] for versions [1.15,1.16,1.17] please perform the backport manually and add the following snippet to your backport PR description:

<details>
	<summary> Overview of commits </summary>
		- <<backport commit 1>>
		- <<backport commit 2>>
		...
</details>

[hc-github-team-consul-core]

@skpratt, a backport is missing for this PR [20545] for versions [1.15,1.16,1.17] please perform the backport manually and add the following snippet to your backport PR description:

<details>
	<summary> Overview of commits </summary>
		- <<backport commit 1>>
		- <<backport commit 2>>
		...
</details>

[hc-github-team-consul-core]

@skpratt, a backport is missing for this PR [20545] for versions [1.15,1.16,1.17] please perform the backport manually and add the following snippet to your backport PR description:

<details>
	<summary> Overview of commits </summary>
		- <<backport commit 1>>
		- <<backport commit 2>>
		...
</details>