Convert pkg/cmd/load_plugins.go to slog

[ritoban23]

What this PR does / why we need it:

Converts pkg/cmd/load_plugins.go to use Go's structured logging package log/slog instead of the standard log package. This improves logging consistency, enables structured logs, and aligns with Helm's modernization efforts.

Closes #31454

Special notes for your reviewer:

• Only internal logging is affected; no user-facing changes.
• All related tests pass.

If applicable:

• [ ] this PR contains user facing changes (the docs needed label should be applied if so)
• [ ] this PR contains unit tests
• [x] this PR has been tested for backwards compatibility