Update browser test Xdebug integration for Xdebug 3

Problem/Motivation

\Drupal\Tests\XdebugRequestTrait::extractCookiesFromRequest() provides Xdebug integration including setting the Xdebug cookie correctly for CLI test-runs. This is based on setting the XDEBUG_CONFIG variable (to something like XDEBUG_CONFIG="idekey=foo") which is what was needed to activate Xdebug on the CLI anyway previously. With Xdebug 3, however, one can set the XDEBUG_SESSION variable directly, similarly to how this works for browser requests (i.e. something loke XDEBUG_SESSION=1). Our Xdebug integration does not pick this up, however, so when working with Xdebug 3 the "old-style" XDEBUG_CONFIG needs to be added as well to make the integration work.

Steps to reproduce

1. Have Xdebug 3 installed and configured
2. Put a breakpoint in the actual test code of a browser test
3. Verify that the breakpoint is hit if running the test with XDEBUG_SESSION=1
4. Now put a breakpoint on a line that is hit during a request that is performed from within the test, for example with drupalGet()
5. Expect the breakpoint to be hit, but be disappointed
6. Add XDEBUG_CONFIG="idekey=1" to the command-line invocation to verify that that does in fact make the runtime breakpoint work

Proposed resolution

Change \Drupal\Tests\XdebugRequestTrait::extractCookiesFromRequest() to support a XDEBUG_SESSION variable in the server variables in addition to the cookies.

Presumably the support for XDEBUG_CONFIG should be kept until Xdebug 2 is no longer supported.

Remaining tasks

User interface changes

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API changes

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Data model changes

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Release notes snippet