doh2: fix CORS by adding `access-control-allow-origin: *`

For old doh we added this in commit a34aa1ee; with the new implementation we somehow forgot.

Better cross-ref: !823 (merged)

added 5 commits

• 85d5f567...bf9bf7bb - 3 commits from branch master
• 4269d3d6 - doh2: fix CORS by adding `access-control-allow-origin: *`
• 597226c0 - doh2 tests: check CORS headers

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approved this merge request

merged

mentioned in commit e3f5e6bc