Drop Task-based execute methods

[martintmk]

Details on the issue fix or feature implementation

After taking a look at this API together we concluded that having ExecuteAsync and ExecuteValueTaskAsync is weird and is to better just expose the native ValueTask based methods.

The downside is the following scenario:

ResilienceStrategy strategy = ...
await strategy.ExecuteAsync(MyExecute);

private async Task MyExecuteAsync(Cancellation token)
{
   // do something
}

However, the caller might as well just do:

ResilienceStrategy strategy = ...
strategy.ExecuteAsync(async cancellationToken => await MyExecute(cancellationToken));

Wdyt, it's ok to drop these?

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[codecov-commenter]

Codecov Report

Merging #1194 (2681445) into main (a4343b7) will decrease coverage by 0.25%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##             main    #1194      +/-   ##
==========================================
- Coverage   83.44%   83.19%   -0.25%     
==========================================
  Files         275      273       -2     
  Lines        6336     6242      -94     
  Branches     1031     1025       -6     
==========================================
- Hits         5287     5193      -94     
  Misses        844      844              
  Partials      205      205              

Flag	Coverage Δ
macos	83.19% <ø> (-0.25%)	⬇️
windows	83.19% <ø> (-0.25%)	⬇️

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Impacted Files	Coverage Δ
...c/Polly.Core/ResilienceStrategy.Async.ValueTask.cs	100.00% <ø> (ø)
.../Polly.Core/ResilienceStrategy.Async.ValueTaskT.cs	100.00% <ø> (ø)