feat: Use v8 for Loader by default for new projects

[mydea]

This PR removes the loader-v8 feature flag, instead enabling this for all orgs. It also adds a new epoch, so that new projects will get v8 by default for the loader. v7 remains selectable for them.

closes getsentry/sentry-javascript#12187

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.24%. Comparing base (7c32da1) to head (ec02194).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #75818      +/-   ##
==========================================
+ Coverage   74.98%   78.24%   +3.25%     
==========================================
  Files        6816     6818       +2     
  Lines      303513   303696     +183     
  Branches    52210    52236      +26     
==========================================
+ Hits       227583   237617   +10034     
+ Misses      69185    59689    -9496     
+ Partials     6745     6390     -355     

Files	Coverage Δ
src/sentry/features/temporary.py	100.00% <ø> (ø)
src/sentry/loader/browsersdkversion.py	85.24% <100.00%> (+2.16%)	⬆️
src/sentry/projectoptions/defaults.py	100.00% <100.00%> (ø)

... and 682 files with indirect coverage changes