Tweak mergify config

[alessio]

Set minimum approvers number to be strictly greater than 1.

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[tac0turtle]

This pull request must be merged manually because it modifies Mergify configuration

[alexanderbez]

Since this is for auto-merge only, I suppose this is OK.

[codecov]

Codecov Report

Merging #5734 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5734   +/-   ##
=======================================
  Coverage   36.22%   36.22%           
=======================================
  Files         335      335           
  Lines       32722    32722           
=======================================
  Hits        11855    11855           
  Misses      19641    19641           
  Partials     1226     1226