fix(prefer-readonly-type): mark rule as deprecated

[RebeccaStevens]

Fixes: #51
Fixes: #150
Fixes: #151
Fixes: #153

Deprecated in favor of prefer-readonly-type-alias and @typescript-eslint/prefer-readonly-parameter-types.

[codecov]

Codecov Report

Merging #246 (fd321ba) into next (2c29cf2) will decrease coverage by 3.48%.
The diff coverage is 73.14%.

❗ Current head fd321ba differs from pull request most recent head af16229. Consider uploading reports for the commit af16229 to get more accurate results

@@            Coverage Diff             @@
##             next     #246      +/-   ##
==========================================
- Coverage   95.41%   91.93%   -3.49%     
==========================================
  Files          35       36       +1     
  Lines         589      682      +93     
  Branches      143      179      +36     
==========================================
+ Hits          562      627      +65     
- Misses         13       32      +19     
- Partials       14       23       +9     

Impacted Files	Coverage Δ
src/common/ignore-options.ts	100.00% <ø> (ø)
src/configs/functional-lite.ts	100.00% <ø> (ø)
src/configs/stylistic.ts	100.00% <ø> (ø)
src/rules/immutable-data.ts	100.00% <ø> (ø)
src/rules/no-return-void.ts	80.00% <63.63%> (-20.00%)	⬇️
src/rules/no-conditional-statement.ts	74.71% <66.15%> (-21.96%)	⬇️
src/util/conditional-imports/tsutils.ts	75.00% <75.00%> (ø)
src/util/typeguard.ts	88.69% <83.33%> (-0.63%)	⬇️
src/index.ts	100.00% <100.00%> (ø)
src/rules/prefer-readonly-type.ts	94.64% <100.00%> (+0.41%)	⬆️
... and 6 more

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[RebeccaStevens]

Closed in favor of #259