Dialog read attribute: don't create a file

[chapulina]

🦟 Bug fix

Summary

The ReadConfigAttribute function was doing more than reading, it was also creating a file if it didn't exist, which wasn't very convenient. Since this is a brand-new function, I think it's ok to change its behavior. I documented it on the migration guide just in case.

• This is needed by Quick start dialog gz-sim#1536

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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[codecov]

Codecov Report

Merging #450 (638c9f1) into ign-gui3 (b543beb) will decrease coverage by 0.16%.
The diff coverage is 80.00%.

@@             Coverage Diff              @@
##           ign-gui3     #450      +/-   ##
============================================
- Coverage     67.97%   67.81%   -0.17%     
============================================
  Files            29       29              
  Lines          3310     3281      -29     
============================================
- Hits           2250     2225      -25     
+ Misses         1060     1056       -4     

Impacted Files	Coverage Δ
src/Dialog.cc	73.01% <80.00%> (-4.16%)	⬇️

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[mayman99]

lgtm