auto dependabot: bump @microsoft/sp-http from 1.15.2 to 1.18.1

Bumps [@microsoft/sp-http](http://aka.ms/spfx) from 1.15.2 to 1.18.1.

---
updated-dependencies:
- dependency-name: "@microsoft/sp-http"
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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