[BUG] README refers to candidate GA build as alpha version

[skkosuri-amzn]

Describe the bug
OpenSearch is an open source search and analytics engine derived from Elasticsearch 7.10.2, and is currently in an alpha state

To Reproduce
Steps to reproduce the behavior:

1. Install the 7/8/2021 candidate GA build of OpenSearch. (used tar version)
2. Open the README.md in the root folder and line-5 refers to this build as alpha version.

Expected behavior
Remove reference to alpha version.

Host/Environment (please complete the following information):

• OS: Linux AL2

[dblock]

This is the README text on 1.0 branch and has been long fixed on main in #853 :(

If we want to fix it we'd need to rebuild core. I suggest punting to 1.x.

[skkosuri-amzn]

Thanks for checking @dblock 1.x Sounds good to me.

[dblock]

Resolved in #950 for 1.x and didn't backport to 1.0.