docs: Add uproot4 writing speedup to v0.6.3 release notes

[matthewfeickert]

Description

Retroactively make mention of the speedup in workspace conversion from JSON to XML + ROOT with the improvements in uproot v4.1.1. This was noticed by @alexander-held and is thanks to @jpivarski fixing a bug in scikit-hep/uproot5#426.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add large speedup of workspace conversion from JSON to XML + ROOT with uproot v4.1.1 to pyhf v0.6.3 release notes
   - c.f. https://github.com/scikit-hep/uproot4/pull/426

[codecov]

Codecov Report

Merging #1601 (348c43b) into master (e72a780) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1601   +/-   ##
=======================================
  Coverage   97.70%   97.70%           
=======================================
  Files          63       63           
  Lines        4050     4050           
  Branches      576      576           
=======================================
  Hits         3957     3957           
  Misses         54       54           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.43% <ø> (ø)
unittests	97.48% <ø> (ø)

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[matthewfeickert]

Relevant RTD build: https://pyhf--1601.org.readthedocs.build/en/1601/release-notes.html#important-notes