Bump Golang version

[matiasinsaurralde]

Changes in this PR

Bumps Golang version to latest stable one: 1.21.4

Also ran go mod tidy.

Why are you making these changes?

Just keeping it up to date and I also think there can be many benefits, for example I explored the new slices package which is recommended and more performant than sort methods in a few scenarios, etc.

Are any changes breaking? (IMPORTANT)

No - tests are fine too

Pre-merge checklist

All of these must be satisfied before this PR is considered ready for merging. Mergeable PRs will be prioritized for review.

• [ ] New packages/exported functions have docstrings.
• [ ] New/changed functionality is thoroughly tested.
• [ ] New/changed functionality has a function giving an example of its usage in the associated test file. See primers/primers_test.go for what this might look like.
• [ ] Changes are documented in CHANGELOG.md in the [Unreleased] section.
• [ ] All code is properly formatted and linted.
• [x] The PR template is filled out.

[matiasinsaurralde]

My only doubt here comes around c5a01d07c26b2e69002f0235e39a67c55fe9f27a, in theory it's properly using the weightedrand method to set a random source however tests fail. Would appreciate any feedback on it.

[TimothyStiles]

@carreter if we're bumping the version then we should probably also bump the go versions that CI/CD runners use. Doing it now before merge.