Dockerfile: up go version to 1.21

[evsamsonov]

Description

Updates golang version in docker image. Fixes issue with generating mocks for go 1.21 invalid go version '1.21.0': must match format 1.23

• Fixes # (issue) https://github.com/vektra/mockery/issues/713

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Version of Golang used when building/testing:

• [ ] 1.11
• [ ] 1.12
• [ ] 1.13
• [ ] 1.14
• [ ] 1.15
• [ ] 1.16
• [ ] 1.17
• [ ] 1.18
• [ ] 1.19
• [ ] 1.20

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (3871351) 74.85912% compared to head (b6f77ea) 74.85912%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #717 +/- ## ============================================= Coverage 74.85912% 74.85912% ============================================= Files 9 9 Lines 2307 2307 ============================================= Hits 1727 1727 Misses 442 442 Partials 138 138 ```

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