fix: use consistent spelling in script_unknown_attribute warning (#18281)

## Summary

The `script_unknown_attribute` warning message used American English
(`Unrecognized`) while every other user-facing error/warning in the
compiler uses British English (`Unrecognised`):

- `options_unrecognised` error: `Unrecognised compiler option…`
- `unknown_code` warning: `is not a recognised code`

This change updates `script_unknown_attribute` to match, then
regenerates `warnings.js` and the reference documentation from the
source message file via `node scripts/process-messages`.

## Test plan
- Spelling change is isolated to the `script_unknown_attribute` warning
message
- `warnings.js` and generated docs were updated by running `node
scripts/process-messages`
- All three occurrences (source, generated JS, generated docs) are
consistent