docs(README): clarify browser support

[pichlermarc]

Which problem is this PR solving?

Currently, browser support of the packages in this repo is not considered production-ready, we should set expectations in the README.md accordingly.

Fixes #4035
See also open-telemetry/opentelemetry.io#3125

Short description of the changes

Adds a not based on open-telemetry/opentelemetry.io#3125 to the README under Browser Support. Removes the checkmark for Browser Support in the supported runtimes

Type of change

• docs

[codecov]

Codecov Report

Merging #4037 (dd98238) into main (5fd656b) will increase coverage by 0.34%.
The diff coverage is n/a.

❗ Current head dd98238 differs from pull request most recent head c57d499. Consider uploading reports for the commit c57d499 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4037      +/-   ##
==========================================
+ Coverage   92.36%   92.71%   +0.34%     
==========================================
  Files         321      287      -34     
  Lines        9264     8143    -1121     
  Branches     1968     1689     -279     
==========================================
- Hits         8557     7550    -1007     
+ Misses        707      593     -114     

see 62 files with indirect coverage changes

[chalin]

👍🏻