RFR: 8291630: Update attribution in webkit.md file

[Kevin Rushforth]

We need to update the attribution in `webkit.md` to reflect the latest WebKit sources and a few missed items.

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Commit messages:
 - 8291630: Update attribution in webkit.md file

Changes: https://git.openjdk.org/jfx/pull/868/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=868&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291630
  Stats: 1152 lines in 1 file changed: 931 ins; 189 del; 32 mod
  Patch: https://git.openjdk.org/jfx/pull/868.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/868/head:pull/868

PR: https://git.openjdk.org/jfx/pull/868