docs: drop deprecated parity-ethereum / openethereum references

[ozpool]

Closes #4018.

The issue is labeled accepted and good first issue. @pappas999's comment on the issue sets the direction:

yes this is still a valid issue, but instead of OE, the docs & code comments etc should be changed to mention geth instead

Both parity-ethereum and its successor openethereum have been unmaintained for years; mentioning them as officially supported execution clients is misleading. This PR drops those references in three places, with intentionally tight scope.

Changes

• README.md — Remove the Parity/Openethereum entry from the Officially supported execution-client list. Geth and Besu remain.
• core/config/docs/chains-evm.toml — The MaxInFlight doc-comment used "geth and parity" as the comparison point for the conservative default of 16. Reword to mention only geth.
• ccip/config/evm/Gnosis_Mainnet.toml — The AuRa-finality comment framed Gnosis Chain finality in terms of parity. Reword to describe AuRa directly and use the current name "Gnosis Chain" instead of the legacy "xDai".

docs/CONFIG.md is generated from the toml comments; left untouched so the build's regen step keeps it in sync.

Out of scope

The tools/docker/ directory still has a docker-compose.paritynet.yaml that's wired up via the compose script. That's functional dev tooling, not a doc reference, so I left it alone — happy to follow up with a separate PR if maintainers want the parity dev images dropped too.

Diff

 .changeset/fix-parity-mentions.md   | +12 (new)
 README.md                           | -1
 ccip/config/evm/Gnosis_Mainnet.toml | +2 -2
 core/config/docs/chains-evm.toml    | +1 -1

Verification

Doc-only change. go build ./... and go vet ./... would not exercise these paths. Local checks:

$ git grep -i 'parity' -- README.md ccip/config/evm/Gnosis_Mainnet.toml core/config/docs/chains-evm.toml
(no output)