doc: fix stale bitcoin_en.xlf reference #35912

doc/release-process.md still instructs users to create the Transifex resource from src/qt/locale/bitcoin_en.xlf, even though that file no longer exists.

It was removed in #34808, which switched the Transifex source to the native Qt .ts file. That PR updated the other references to .xlf in this document, but this one was missed. The last step in the same list already refers to bitcoin_en.ts, as does .tx/config.

This patch updates the remaining outdated reference.

Docs-only change. No tests run.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35912.

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Concept ACK.

ACK e98ffd4bd82d35ea316f977a9659c8080066b643. This was overlooked in #34808.