[BUG] Updates needed in DEVELOPER_GUIDE.md, TESTING.md, etc.

[ahopp]

Describe the bug
There are some documentation issues that need to be addressed in the repo docs.

• In DEVELOPER_GUIDE.md under the heading 'Project Layout', it mentions that 'docs' contains the documentation for the project. This is incorrect as that folder no longer exists.
• In CONTRIBUTING.md, "please see OpenSearch Docs repo." points to README.md but I think it should point to here since the docs folder no longer exists.
• The MAINTAINERS.md lists 13 maintainers and points to this file which lists 2. If there is a difference we should make it clear, if not we should should correct.
• In TESTING.md under the heading "Testing Backwards Compatibility" it references x-pack in an example e.g. "specific tests from the x-pack rolling upgrade from 7.7.0".
• RELEASING.md has blank/empty TODO at the bottom.
• In TESTING.md under the heading "Testing Backwards Compatibility" it references the link to VersionCollection which is 404ing.
• In TESTING.md under the heading "Refactor code to make it easier to test" it references an Elastic issue. We should remove or update to an OpenSearch example.
• In TESTING.md under the heading "Test coverage analysis" it says "Test coverage reporting used to be possible with JaCoCo when OpenSearch was using Maven as its build system" but this is inaccurate. OpenSearch never used maven. This paragraph also mentions an elastic issue. I would suggest dropping the whole paragraph.
• We lowered the requirements for Java from 14 to 11. This doc should be updated.
• In TESTING.md under the heading "Building with extra plugins" 'opensearch-extra' should be formatted as inline code.
• In DEVELOPER_GUIDE.md under the heading 'Project Layout', it mentions that 'plugins' contains the 'Officially supported plugins to OpenSearch' - while it might be meaningful for ODFE plugins, it's confusing for OpenSearch.
• In client/benchmark/README.md references a document on benchmarks.elasticsearch.org.s3.amazonaws.com.

Lastly, TESTING.md references Rally - I'm not sure that OpenSearch will work with Rally and we should change this to OpenSearch Benchmarks as soon as it is released. My suggestion would be to drop the benchmarking section until OpenSearch Benchmarks is out.

To Reproduce
Open repo docs.

Expected behavior
Listed above

Plugins
N/A

Screenshots
N/A

Host/Environment (please complete the following information):

• OS: N/A
• Version: N/A

Additional context
None.

[dblock]

@ahopp I think you almost have the diff for this :) PR?

[dblock]

I'll take this.

[mch2]

@ahopp We lowered the requirements for Java from 14 to 11 only on the Main branch, this has not been backported to 1.x or 1.0.

[ahopp]

I'll take this.

I appreciate it db!

[dblock]

Did in #946. There are a couple of open items that belong in the .github repo, I opened opensearch-project/.github#20.

[skkosuri-amzn]

Tagging: README issue: #948