Use errgroup consistently in MultiWrite

[imjasonh]

#958 added support for WithJobs to remote.Write, etc., and added context timeout/cancellation handling (correctly, we think) at the same time.

This change ports that context handling to remote.MultiWrite (correctly, we think). If it's not correct, it's at least consistently incorrect.

[codecov-io]

Codecov Report

Merging #966 (20608e4) into main (bea57ad) will decrease coverage by 0.01%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##             main     #966      +/-   ##
==========================================
- Coverage   73.13%   73.11%   -0.02%     
==========================================
  Files         107      107              
  Lines        4828     4832       +4     
==========================================
+ Hits         3531     3533       +2     
- Misses        780      782       +2     
  Partials      517      517              

Impacted Files	Coverage Δ
pkg/v1/remote/multi_write.go	57.01% <75.00%> (-0.26%)	⬇️

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