NEWS: add release notes for v0.59.0

[garlick]

Here are some proposed release notes for flux-core v0.59.0 that assume the following are merged

• pmi: prepend Flux PMI directory to LD_LIBRARY_PATH #5715
• shell: add shell.post-init plugin calllback topic between shell.init and first task.init #5719

[codecov]

Codecov Report

Merging #5720 (ae753ff) into master (d8e288c) will decrease coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5720      +/-   ##
==========================================
- Coverage   83.46%   83.45%   -0.01%     
==========================================
  Files         486      486              
  Lines       82933    82933              
==========================================
- Hits        69218    69211       -7     
- Misses      13715    13722       +7     

see 7 files with indirect coverage changes

[garlick]

Added

• shell: write hwloc XML to a file with HWLOC_XMLFILE set with -o hwloc.xmlfile #5721

[grondo]

Force-pushed an update to today's date and LGTM!

[garlick]

Setting MWP!