refactor: set minimal go version to v1.21

[muktihari]

• Set minimal Go version to v1.21 and drop golang.org/x/exp dependency as slices package is available in v1.21 and we can define our own constaints for numeric values without depend on that package.
• Can not drop golang.org/x/test since we still don't have replacement to make Title Case (we use this package since strings.Title has been deprecated). Unit test for strutil.ToTitle is added in case in the future we want to create our own simplified implementation, we should pass the test.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (2a825d2) to head (95656d6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #420 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 41 41 Lines 3629 3629 ========================================= Hits 3629 3629 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.