golang: Update golang to 1.21.1

This version includes security fixes which address the following CVEs:

* CVE-2023-39318
* CVE-2023-39319
* CVE-2023-39320
* CVE-2023-39321
* CVE-2023-39322

Also changes to internal/prometheus/TestJSONSerialization: add new metric

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>