Drop IE11 support

[nlopin]

After the discussion in the internal channel we decided to remove support for old browsers, especially IE11.

Reasons:

• it is not supported by any big tech company anymore, hence there was a "force" migration to newer browsers
• our main source of IE users is B2B. According to their statistics, WebBooker only has 0.09% IE11 users and the b2b authorization does not have IE11 users at all

As a result, we need to review the dependencies we use to provide the support, remove it and update the documentation. I suspect it will mostly mean checking what polyfills we include in the bundle.

The future impact is that we can use modern API without increasing the bundle size.

[martimalek]

I feel like this may be done already with the work in #321, since there the polyfills.ts file was removed, I'm not sure if we are still lacking something. @nlopin do you think this was everything we needed?