chore(build): Bumps up Makefile required go version to 1.21

[keramosg]

Versioning Fix

Description

Per determined/master/go.mod the version said module expects is 1.21 but Makefile still required 1.20 in its pre-build checks.

Test Plan

1. Run make all with go1.21.x
2. Check if it successfully builds

Checklist

• [ ] Changes have been manually QA'd
• [ ] User-facing API changes need the "User-facing API Change" label.
• [ ] Release notes should be added as a separate file under docs/release-notes/. See Release Note for details.
• [ ] Licenses should be included for new code which was copied and/or modified from any external code.

Ticket

DET-9791

[cla-bot[bot]]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have the users @keramosg on file. In order for us to review and merge your code, please start the CLA process at https://determined.ai/cla.

After we approve your CLA, we will update the contributors list (private) and comment @cla-bot[bot] check to rerun the check.

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[cla-bot[bot]]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have the users @keramosg on file. In order for us to review and merge your code, please start the CLA process at https://determined.ai/cla.

After we approve your CLA, we will update the contributors list (private) and comment @cla-bot[bot] check to rerun the check.

[keita-determined]

didnt notice this PR, but the same fix is merged as #7751