breaking(transport): Make HTTP2Transport the default

[BYK]

Making the HTTP2 transport the default when h2 and httpcore packages are installed. We've been testing this on Sentry SaaS for a while without any issues.

We should promote installing the SDK as sentry-sdk[http2] for this to be picked up though. Since we still have to support Python 3.7 and h2 not being supported there, we cannot install it by default and use HTTP2 directly.

---

EDIT: please merge after potel dogfooding

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 84.85%. Comparing base (38cc2a8) to head (545bbe9). :warning: Report is 1 commits behind head on potel-base. :white_check_mark: All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff               @@
##           potel-base    #4492      +/-   ##
==============================================
+ Coverage       84.75%   84.85%   +0.09%     
==============================================
  Files             158      158              
  Lines           16093    16093              
  Branches         2564     2564              
==============================================
+ Hits            13640    13656      +16     
+ Misses           1660     1648      -12     
+ Partials          793      789       -4     

Files with missing lines	Coverage Δ
sentry_sdk/consts.py	99.06% <ø> (ø)
sentry_sdk/transport.py	83.90% <100.00%> (-0.21%)	:arrow_down:

... and 3 files with indirect coverage changes

[BYK]

The test failures look unrelated and consistent with each other.

[sl0thentr0py]

ty @BYK, will take over and fix the tests.

will also make this a draft till we're ready to merge!