chore: deprecate object syntax for proxy

[rishabh3112]

• [ ] This is a bugfix
• [ ] This is a feature
• [ ] This is a code refactor
• [ ] This is a test update
• [ ] This is a docs update
• [x] This is a metadata update

For Bugs and Features; did you add new tests?

Will do if required.

Motivation / Use-Case

#2454 https://github.com/webpack/webpack-dev-server/issues/2454#issuecomment-812059043

Breaking Changes

Additional Info

[codecov[bot]]

Codecov Report

Merging #3224 (a5c93c5) into master (1997a6e) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3224   +/-   ##
=======================================
  Coverage   95.19%   95.19%           
=======================================
  Files          37       37           
  Lines        1248     1249    +1     
  Branches      352      352           
=======================================
+ Hits         1188     1189    +1     
  Misses         54       54           
  Partials        6        6           

Impacted Files	Coverage Δ
lib/Server.js	94.56% <100.00%> (+0.01%)	:arrow_up:

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