Bump is-svg, postcss-svgo and imagemin-svgo in /website

Bumps [is-svg](https://github.com/sindresorhus/is-svg), [postcss-svgo](https://github.com/cssnano/cssnano) and [imagemin-svgo](https://github.com/imagemin/imagemin-svgo). These dependencies needed to be updated together.

Updates `is-svg` from 3.0.0 to 4.3.2
- [Release notes](https://github.com/sindresorhus/is-svg/releases)
- [Commits](sindresorhus/is-svg@v3.0.0...v4.3.2)

Updates `postcss-svgo` from 4.0.2 to 4.0.3
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](cssnano/cssnano@4.0.2...4.0.3)

Updates `imagemin-svgo` from 7.0.0 to 7.1.0
- [Release notes](https://github.com/imagemin/imagemin-svgo/releases)
- [Commits](imagemin/imagemin-svgo@v7.0.0...v7.1.0)

---
updated-dependencies:
- dependency-name: is-svg
  dependency-type: indirect
- dependency-name: postcss-svgo
  dependency-type: indirect
- dependency-name: imagemin-svgo
  dependency-type: indirect
...

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

Author: dependabot[bot]