[Bug] Opt out not compatible with some older browsers about matomo HOT 2 OPEN

I've created a PR to fix the problem with const only, as that clearly breaks on older browsers. But we still need to do some testing on older browsers to see if everything else works correctly for the opt out.
As part of this issue we should also consider adding tests for some older browsers, as we are currently only running test with latest chrome and will not see such problems early enough.

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As part of this work, it would great to define what older browsers we want to support for the tracker and/or what does 'on best effort basis' mean.
With "we try to support as much as we can" there's no boundary and someone can always say "this doesn't work for me" or "we can't use that".
Based on https://caniuse.com/const const is available to 97% percent of users.

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