chore: add linter that catches math/rand imports

[mauri870]

Proposed commit message

This is a follow up to #42025 that migrated from math/rand to the newer math/rand/v2 package. Currently there is nothing preventing contributors to commiting code using math/rand and regressing these changes.

There is still code in beats that use math/rand, but it is mainly in tests. Since the linter only runs on new code I'm not fixing it here.

Introduce a linter that catches math/rand imports in Go files.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Relates chore: add linter that denies importing math/rand elastic-agent#6650
• Relates feat(libbeat): use math/rand/v2 and drop seed init #42025

Logs

Offending code gets warned like this:

filebeat/input/filestream/internal/task/group_test.go:24:2: import 'math/rand' is not allowed from list 'main': superseded by math/rand/v2 (depguard)
        "math/rand"

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @mauri870? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.