Add a validator for the manifest version

[FlorentClarret]

What does this PR do?

Stop running the v2 validators for manifest v1 files and add a validator for the version.

Motivation

We are only running v2 validators:

integrations-core/datadog_checks_dev/datadog_checks/dev/tooling/manifest_validator/__init__.py

Line 8 in 2d86725

return get_v2_validators(ctx, is_extras, is_marketplace)

Because of that, I got the following trying to validate a v1 file (that was created a few weeks ago on a branch):

  File "/Users/florent.clarret/go/src/github.com/DataDog/integrations-core/datadog_checks_dev/datadog_checks/dev/tooling/manifest_validator/common/validator.py", line 102, in validate
    if not maintainer.isascii():
AttributeError: 'NoneType' object has no attribute 'isascii'

With this fix, we have:

╰─❯ ddev validate manifest impala                                                                                                                                                                                                                                                                                       ─╯
Validating manifest.json files for 1 checks ...
impala/manifest.json... FAILED
Manifest version must be >= 2.0.0
1 invalid files

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #12788 (42dfbf2) into master (2846ea4) will not change coverage.
The diff coverage is 60.00%.

Flag	Coverage Δ
avi_vantage	91.92% <ø> (ø)
boundary	100.00% <ø> (ø)
calico	83.33% <ø> (ø)
ddev	97.99% <ø> (ø)

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