Fixes for unit tests and golangci-lint

[mrtamm]

This PR contains minor fixes to unit tests and the golangci-lint configuration. However, it does not cover the issues reported by the linter (to keep this PR small).

Unit tests:

1. storage tests: provided valid Google Storage URL for testing download
2. storage tests: fixed configuration tests that failed as htsget configuration was not explicitly disabled
3. core test: added more precise logging as the test occasionally failed (but not anymore)

golangci-lint:

1. had to upgrade some go dependencies and go version (to 1.21) as the linter failed to handle some dependencies properly, and thus failed.
2. also updated other GitHub actions.

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Follow-up actions after this PR as separate PRs:

1. fix problems reported by the linter
2. upgrade dependencies

[mrtamm]

Here is the current status of the GitHub actions workflow: https://github.com/mrtamm/funnel-gdi/actions/runs/8067628969