Bump google-api-core from 1.5.1 to 2.4.0

Bumps [google-api-core](https://github.com/googleapis/python-api-core) from 1.5.1 to 2.4.0.
- [Release notes](https://github.com/googleapis/python-api-core/releases)
- [Changelog](https://github.com/googleapis/python-api-core/blob/main/CHANGELOG.md)
- [Commits](https://github.com/googleapis/python-api-core/compare/v1.5.1...v2.4.0)

---
updated-dependencies:
- dependency-name: google-api-core
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

requirements.txt

@@ -4,7 +4,7 @@ certifi==2018.10.15
 chardet==3.0.4
 elasticsearch==6.3.1
 falcon==1.4.1
-google-api-core==1.5.1
+google-api-core==2.4.0
 google-auth==1.5.1
 google-cloud-core==0.28.1
 google-cloud-datastore==1.7.1