Increase min supported tensorflow to 1.14.0

[chriselion]

Proposed change(s)

Increase min supported tensorflow to 1.14.0 - this version was released in June 2019.
Reduce number of test configs (which was the main goal).

Previously we ran

• Python 3.6.1 with TF 1.7.0
• Python 3.7.3 with TF 1.15.x
• Python 3.7.3 with TF 2.2.0 RC1
• Python 3.8.2 with TF 2.2.0 RC1

There's not a lot of benefit to running both of the last two, so we'll get rid of the "Python 3.7.3 with TF 2.2.0 RC1" variant.
The updated list is

• Python 3.6.1 with TF 1.14.0
• Python 3.7.3 with TF 1.15.x
• Python 3.8.2 with TF 2.3.0

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments

[chriselion]

Moved this to the current "unreleased" section.