7.9.2 release

[nkammah]

This PR prepare 7.9 branch for 7.9.2 release:

• bump chart and Docker images version from 7.9.2-SNAPSHOT to 7.9.2
• remove notices on README that this is a development branch and development branch specific install instructions

Note

• remove notices on README that this is a development branch and development branch specific install instructions

This is required because released Helm charts will be packaged from this PR commit including the README files that will be available inside the chart packages and in Helm Hub pages (example: Elasticsearch chart page).

This change will be reverted in the next PR after release when we will bump 7.9 branch to 7.9.3-SNAPSHOT version.

[mgreau]

LGTM

[nkammah]

7.9.2 - 2020/09/24

• 7.9.2 as the default stack version
• Bump Helm version to 2.16.12 (@jmlrt)

Elasticsearch

PR	Author	Title
#729	@floretan	Include pre-releases in the semver range.
#810	@luanguimaraesla	Add loadBalancer externalTrafficPolicy option
#778	@jmlrt	Fix secrets names in examples

Metricbeat

PR	Author	Title
#778	erihanse	Support secrets

[jmlrt]

Missing these changes: https://github.com/elastic/helm-charts/pull/803/files#diff-04c6e90faac2675aa89e2176d2eec7d8L106-R117