fix: log browser url to stderr instead of stdout

[marceljk]

Description

relates to #1125

Testing instructions

1. Checkout the branch of this PR and run $ make build
2. Set the session limit to 1s, to simulate an expired session
   $ ./bin/stackit config set --session-time-limit 1s
3. Authenticate with your user account
   $ ./bin/stackit auth login
4. Run a command with output format json, e.g.:
   $ ./bin/stackit network list -o json | jq
   -> should work
5. Checkout the main branch and rerun the previous command
   $ ./bin/stackit network list -o json | jq
   -> should fail

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see e.g. here)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[cgoetz-inovex]

lgtm, manual test successfull

[github-actions]

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/stackit-cli/internal/pkg/auth	56.52% (ø)

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/stackit-cli/internal/pkg/auth/user_login.go	9.89% (ø)	182	18	164

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.