Pin mike at version 0.5.1

[masci]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests before creating one)
• The PR follows our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce?

It pins mike version at 0.5.1 in the requirements file

• What is the current behavior?

mike version is not pinned and it gets the latest at each docs build. We're having a problem with latest version 0.5.2 as seen here https://github.com/arduino/arduino-cli/runs/716092672#step:11:103

• What is the new behavior?

While we investigate why 0.5.2 fails, pin 0.5.1 that was proven to work

• Does this PR introduce a breaking change?

No CLI changes

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