Update minimum golang version to v1.21.7

[dhrubabasu]

Why this should be merged

go1.22 was released. Golang only provides security patches for the prior version so we'll update to go1.21.7.

How this works

• Update to go1.21.7
• Use the new min and max built-in functions instead of our math.Max and math.Min functions.
• Use the new slices pkg instead of /x/exp/slices.
• Use the new maps pkg instead of /x/exp/maps.
  • maps.Keys and maps.Values were removed in go1.21 (maps: remove Keys and Values for Go 1.21 golang/go#61538) so we are still using the exp pkg in some files
• Use the new cmp pkg instead of /x/exp/constraints

How this was tested

CI