Update parallel algorithms to use cpp20 concepts

[sidhikumari5]

[StellarBot]

Can one of the admins verify this patch?

[codacy-production[bot]]

Coverage summary from Codacy

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Coverage variation	Diff coverage
Report missing for 3f0acd7a4737eb2ef64132ad4fab1560ac6e53c3[^1]	:white_check_mark: 0.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (3f0acd7a4737eb2ef64132ad4fab1560ac6e53c3)	Report Missing	Report Missing	Report Missing
Head commit (b491fc634d1498880196b4aeec7731a0c0ca844f)	57287	0	0.00%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#6700)	8	0	0.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[^1]: Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct.

[hkaiser]

Very nice!

[hkaiser]

@sidhikumari5 Congratulations! Your PR was merged just now to the HPX master branch.