chore: Bump directory-size-exporter to use Golang 1.21.0

[chrkl]

Description

Changes proposed in this pull request:

• Bump directory-size-exporter to use Golang 1.21.0

Related Issues and Documents

Closes:

Related issues:

Traceability

• The PR is linked to a GitHub Issue.
• New features have a milestone label set.
• New features have defined acceptance criteria in a corresponding GitHub Issue, and all criteria are satisfied with this PR.
• The corresponding GitHub Issue has a respective area label.
• The follow-up issues (if any) are linked in the Related Issues section.

Testability

The feature is unit-tested:

• Yes.
• No, because unit tests are not needed.
• No, because of ...

The feature is e2e-tested:

• Yes.
• No, because e2e-tests are not needed.
• No, because of ...

Tests conducted for the PR:

Codebase

• My code follows the Effective Go style guidelines.
• The code was planned and designed following the defined architecture and the separation of concerns.
• The code has sufficient comments, particularly for all hard-to-understand areas.
• This PR adds value and shows no feature creep.
• I have augmented the test suite that proves my fix is effective or that my feature works.
• Adjusted the documentation if the change is user-facing.