chore(deps): bump d3-svg-legend in /superset-frontend

Bumps [d3-svg-legend](https://github.com/susielu/d3-legend) from 1.13.0 to 2.25.6.
- [Release notes](https://github.com/susielu/d3-legend/releases)
- [Commits](susielu/d3-legend@v1.13.0...v2.25.6)

---
updated-dependencies:
- dependency-name: d3-svg-legend
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

superset-frontend/plugins/legacy-plugin-chart-heatmap/package.json

@@ -23,7 +23,7 @@
   ],
   "dependencies": {
     "d3": "^3.5.17",
-    "d3-svg-legend": "^1.x",
+    "d3-svg-legend": "^2.x",
     "d3-tip": "^0.9.1",
     "prop-types": "^15.6.2"
   },