Update engines, @types/node to Node.js v12, add changelog entry

[ramya-rao-a]

This PR is the last leg of the work being done as part of #7022

Net changes:

• package.json file
  • Update engines field to >=12
  • Update @types/node to `^12.0.0
  • Increment minor version if the package has gone GA before
• linter
  • Update the eslint rule around the engines field to check for 12 instead of 8
  • Remove the turning off of the above eslint rule where people could not support Node.js 8
• Add changelog entry if the package has gone GA before.
  • I used the "features" section as am treating the use of "new types" as a feature.

Other related work we already merged:

• Remove Node v8 from run matrix #15647
• Update Node.js & browser support in README #16011
• Prepare to move to @types/node v12 #16012
• Prepare Storage packages to move to @types/node v12  #16020

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[deyaaeldeen]

TA is looking good!

[xirzec]

Wow! What an incredible effort. Thank you for your diligence!

I will admit it feels a little bit odd to place the release note under 'features added', but I'm not sure where else it would go.

[ellismg]

EventGrid changes look great, thanks for doing this, Ramya!

[richardpark-msft]

No version bump for this legacy package?

[ramya-rao-a]

Nah, no plans on releasing anything here.

[richardpark-msft]

service-bus, event-hubs and monitor-query look good! This is awesome!

[chradek]

Looks good! I was worried about the engines-strict setting but it is set to false by default and otherwise is just advisory, so it seems worth the push.