[jfx26u] Integrated: Merge jfx:jfx26

[Kevin Rushforth]

Clean merge from `jfx:jfx26` to `jfx26u`.

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Commit messages:
 - Merge remote-tracking branch 'jfx/jfx26' into merge-jfx-jfx26-to-master-2026-02-24
 - Merge
 - 8377393: RichTextArea: failed to export null color attribute value
 - 8377099: Additional WebKit 623.1 fixes from WebKitGTK 2.50.4
 - Merge
 - 8376282: [linux, macos] JavaFX fails to build WebKit in DebugNative
 - 8375225: WebIObserverTest fails with WebKit 623.1
 - 8368572: Update WebKit to 623.1
 - 8304008: Update README.md and CONTRIBUTING.md for jfx update repos
 - 8376022: Change JavaFX release version to 26.0.1 in jfx26u

The merge commit only contains trivial merges, so no merge-specific webrevs have been generated.

Changes: https://git.openjdk.org/jfx26u/pull/11/files
  Stats: 506493 lines in 9009 files changed: 254359 ins; 189225 del; 62909 mod
  Patch: https://git.openjdk.org/jfx26u/pull/11.diff
  Fetch: git fetch https://git.openjdk.org/jfx26u.git pull/11/head:pull/11

PR: https://git.openjdk.org/jfx26u/pull/11