Upgrade ajv to v7

[pgrzesik]

Addresses #8695

This PR aims to bring ajv v7 upgrade whenever we feel it's good time for it. In includes two commits that were reverted with https://github.com/serverless/serverless/commits/master as well as picks up the work that was done as a part of #8742.

[codecov]

Codecov Report

Merging #8779 (5d2b332) into master (b23bfdb) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #8779      +/-   ##
==========================================
+ Coverage   87.74%   87.75%   +0.01%     
==========================================
  Files         264      264              
  Lines        9780     9778       -2     
==========================================
  Hits         8581     8581              
+ Misses       1199     1197       -2     

Impacted Files	Coverage Δ
lib/Serverless.js	97.36% <100.00%> (ø)
lib/classes/ConfigSchemaHandler/index.js	91.25% <100.00%> (-0.11%)	⬇️
.../classes/ConfigSchemaHandler/normalizeAjvErrors.js	95.72% <100.00%> (-0.08%)	⬇️
.../classes/ConfigSchemaHandler/resolveAjvValidate.js	100.00% <100.00%> (ø)
lib/classes/Service.js	86.06% <100.00%> (ø)

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[mnapoli]

Following #10440 we can probably close this one?

[pgrzesik]

Definitely @mnapoli