rename api.md to old_api.md and update links (to point to new_api.md)

[reyang]

Based on the comment from @arminru, I'm splitting the PR to two parts.

This is the 1st part. After this PR got merged, I will update #1672 which will be the 2nd part.

Previously we've decided to make progress on the metrics API spec by creating a separate new_api.md doc just to make PR review easier.

Now that the metrics API spec is getting in shape, we've discussed in the Apr. 29th Metrics API/SDK spec SIG meeting that we will rename it to api.md in early May.

I decided to keep the previous spec for now (by renaming it to old_api.md) so it won't confuse GitHub PR diff.

Changes

• Renamed api.md to old_api.md.
• Made necessary updates to the links.

Related oteps

• https://github.com/open-telemetry/oteps/blob/main/text/metrics/0146-metrics-prototype-scenarios.md