feat!: enable h2 by default

[metcoder95]

This relates to...

This enables HTTP/2 support by default in Undici.

Rationale

Currently, users must explicitly opt-in to H2 support via allowH2: true. This change adjusts Undici's defaults to align with current web standards and improve out-of-the-box performance.

Changes

This PR changes the default value of the allowH2 option from false to true in the connection builder (lib/core/connect.js). When connecting to HTTPS servers, Undici will now use HTTP/2 if the server advertises it through ALPN negotiation, falling back to HTTP/1.1 if not supported.

Features

• HTTP/2 is now enabled by default for connections to servers that support it via ALPN negotiation
• Users can still explicitly disable H2 by setting allowH2: false in client options

Bug Fixes

N/A

Breaking Changes and Deprecations

Breaking Change: The default value of allowH2 changes from false to true.

Impact:

• Applications relying on HTTP/1.1-only behavior by default will now use HTTP/2 when connecting to H2-capable servers
• Users who need HTTP/1.1-only connections must now explicitly set allowH2: false in their client configuration
• The protocol upgrade is transparent and should not affect most applications, but may change connection behavior and performance characteristics

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.26%. Comparing base (393094a) to head (b55ff3b).

Additional details and impacted files

@@           Coverage Diff           @@
##             next    #4828   +/-   ##
=======================================
  Coverage   93.26%   93.26%           
=======================================
  Files         107      107           
  Lines       34031    34031           
=======================================
  Hits        31738    31738           
  Misses       2293     2293           

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[mcollina]

lgtm

[mcollina]

@metcoder95 can you look into the failure? It seems relevant.