Bump elliptic from 6.4.0 to 6.5.4 in /kibana/plugins/demo

Bumps [elliptic](https://github.com/indutny/elliptic) from 6.4.0 to 6.5.4.
- [Release notes](https://github.com/indutny/elliptic/releases)
- [Commits](indutny/elliptic@v6.4.0...v6.5.4)

Signed-off-by: dependabot[bot] <[email protected]>