ci: disable tensorflow 2.5-2.7 tests.

[ioga]

Description

A bunch of nightlies have been failing due to shortage of k80. I believe we can omit TF tests for 2.5, 2.6, 2.7 to reclaim some capacity.

• Real users now want newer versions of TF, not these.
• Tests for 2.4 (our "legacy") and 2.8 (our "current") should cover functionality and compatibility well enough. These tests have never caught any issues past the initial release.
• IIRC all our EE TF users are already on TF 2.8+

Test Plan

Commentary (optional)

Checklist

• Changes have been manually QA'd
• User-facing API changes need the "User-facing API Change" label.
• Release notes should be added as a separate file under docs/release-notes/.
  See Release Note for details.
• Licenses should be included for new code which was copied and/or modified from any external code.

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[mpkouznetsov]

Reminds me that we should stop building those environments.