docs(toolkit-lib): update README to use getStackByName for retrieving stack template

[fossamagna]

Now because getStack is deprecated. I replaced getStack by getStackByName.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[mrgrain]

Thank you!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.09%. Comparing base (a05a764) to head (fda53bb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #558   +/-   ##
=======================================
  Coverage   79.09%   79.09%           
=======================================
  Files          47       47           
  Lines        7074     7074           
  Branches      791      791           
=======================================
  Hits         5595     5595           
  Misses       1460     1460           
  Partials       19       19           

Flag	Coverage Δ
suite.unit	79.09% <ø> (ø)

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