Rename QueueEvent::StartWork (#6015)

# Description

When we send `QueueEvent::StartWork`, we have already completed the
execution. This may be a leftover of a previous logic change. Currently,
the name is misleading, so it would be better to rename it to
`FinishWork`.


https://github.com/paritytech/polkadot-sdk/blob/c52675ef/polkadot/node/core/pvf/src/execute/queue.rs#L632-L646


https://github.com/paritytech/polkadot-sdk/blob/c52675ef

/polkadot/node/core/pvf/src/execute/queue.rs#L361-L363

Fixes https://github.com/paritytech/polkadot-sdk/issues/5910

## Integration

Shouldn't affect downstream projects.

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Co-authored-by: GitHub Action <[email protected]>