Bump Elasticsearch.Net.v6 (#489)

Bumps [Elasticsearch.Net.v6](https://github.com/elastic/elasticsearch-net) from 6.9.0-ci20190620T132817 to 6.9.0-ci20210419T072245.
- [Release notes](https://github.com/elastic/elasticsearch-net/releases)
- [Changelog](https://github.com/elastic/elasticsearch-net/blob/main/CHANGELOG.md)
- [Commits](https://github.com/elastic/elasticsearch-net/commits)

---
updated-dependencies:
- dependency-name: Elasticsearch.Net.v6
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

test/Serilog.Sinks.Elasticsearch.IntegrationTests/Serilog.Sinks.Elasticsearch.IntegrationTests.csproj

@@ -14,7 +14,7 @@
   </ItemGroup>
   <ItemGroup>
     <DotNetCliToolReference Include="dotnet-xunit" Version="2.3.1" />
-    <PackageReference Include="Elasticsearch.Net.v6" Version="6.9.0-ci20190620T132817" />
+    <PackageReference Include="Elasticsearch.Net.v6" Version="6.9.0-ci20210419T072245" />
     <PackageReference Include="NEST.v6" Version="6.9.0-ci20190620T132817" />
     <PackageReference Include="Serilog.Sinks.Console" Version="4.1.0" />