Support gym 0.26 in week6 seminar

[q0o0p]

There are changes in gym API between 0.25 and 0.26 These changes break backward compatibility. Also there are some changes with video recording in gym 0.23 or so.

In this commit we updated the code of week6 seminar to use gym 0.26

What is changed:

• env initializer;
• env.render;
• env.reset;
• env.step;
• video recording

Originally I wanted to add if-else instructions everywhere to support all versions of gym, but the notebook become messy, so let's support just 0.26 and higher.

Note: didn't test in Colab, only locally

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[anna-vb]

Upd: tested in Colab, works fine too