Remove deprecated appendToStream - they're new on v7

[chapulina]

Follow-up to

• 👩‍🌾 Prevent -0 with out stream operator #206
• Fix output stream operator #376

Summary

The appendToStream functions were added on main, so they don't need to be deprecated and can be removed.

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 #454 (ac5e949) into main (3827f4c) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #454      +/-   ##
==========================================
- Coverage   99.68%   99.68%   -0.01%     
==========================================
  Files          73       73              
  Lines        6639     6631       -8     
==========================================
- Hits         6618     6610       -8     
  Misses         21       21              

Impacted Files	Coverage Δ
include/gz/math/Helpers.hh	100.00% <ø> (ø)

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