[ci] Test against opensearch 1.2.x

[phillbaker]

Description

Moves testing support forward to 1.2.x, see #44

Issues Resolved

Part of 2.x support

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[prudhvigodithi]

Hey @phillbaker this should be merged to 1.x branch and also should we change image version to 1.3? as that is the supported 1.x version.
Also this entire check - os-major-version: "1-opensearch" should be excluded from main and should remain only in 1.x branch. This was the main can be fast forward and deal with only upcoming 2.x version releases.
Please check https://github.com/opensearch-project/terraform-provider-opensearch#version-and-branching and add your thoughts.
@bbarani @peterzhuamazon

[phillbaker]

See my comment in #44 (comment), I'll have a follow up PR for 1.3.x support coming next. The intent is to break up the work into chunks, although this one didn't require any specific changes.

[prudhvigodithi]

Closing this PR as the main branch CI checks are tested with 2.x versions, related PR.
@phillbaker please feel free to re-open if required.
@bbarani @peterzhuamazon