[DOCS] Remove dates from release notes

[jrodewig]

Problem

Duplicating the release date in the Elasticsearch-Hadoop release notes (RNs) creates some potential issues:

• It's possible for the RNs to erroneously drift from the dates on https://www.elastic.co/downloads/past-releases#es-hadoop, which are regarded as the source of truth.
• It puts extra burden on the docs coordinator to update and publish ES-Hadoop RNs on the day of the release. If we choose to remove dates, we can safely publish release notes before release.
• The inclusion of release dates is inconsistent with RNs for other Elastic solutions and products.

Proposal

• Do not include release dates for Elasticsearch-Hadoop RNs going forward.
• Remove release dates from any RNs on the master, 7.x, 7.5, and 6.8 branches. Earlier release notes will be left as-is.

[jrodewig]

Closed by #1396, #1397, and #1398.