Replace pika::merge with std::merge

[aurianer]

No description provided.

[msimberg]

Could you update the title please?

[aurianer]

So I ran the problematic miniapp with the specific configuration (see the comment of the gist for the exact options) and I got 2 segmentation faults on 86 iterations, each of 20 runs for the std::merge version and I got 1 segmentation fault in 72 iterations of 20 runs for the pika::merge version.
https://gist.github.com/aurianer/e73bdbdf644f10fbbd7e242e5b545702
https://gist.github.com/aurianer/0f8f7e3b9280b45c4ff363a649226793
So there is a problem but it's unlikely to be triggered by this PR

[aurianer]

cscs-ci run

[codecov-commenter]

Codecov Report

Merging #977 (e499a8f) into master (a9b539d) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master     #977      +/-   ##
==========================================
- Coverage   93.97%   93.94%   -0.04%     
==========================================
  Files         143      143              
  Lines        8682     8684       +2     
  Branches     1113     1113              
==========================================
- Hits         8159     8158       -1     
- Misses        336      338       +2     
- Partials      187      188       +1     

Files Changed	Coverage Δ
...af/eigensolver/tridiag_solver/index_manipulation.h	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes