Rename ExecuteCoreAsync to ExecuteCore

[martintmk]

Details on the issue fix or feature implementation

For context:
#1233 (comment)

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]

Codecov Report

Merging #1424 (6585540) into main (7199f75) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1424   +/-   ##
=======================================
  Coverage   83.92%   83.92%           
=======================================
  Files         275      275           
  Lines        6526     6526           
  Branches     1007     1007           
=======================================
  Hits         5477     5477           
  Misses        840      840           
  Partials      209      209           

Flag	Coverage Δ
linux	?
macos	83.92% <100.00%> (ø)
windows	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/Polly.Core/NullResilienceStrategy.cs	100.00% <ø> (ø)
...rc/Polly.Core/Timeout/TimeoutResilienceStrategy.cs	100.00% <ø> (ø)
src/Polly.Core/Utils/OutcomeResilienceStrategy.cs	100.00% <ø> (ø)
src/Polly.Core/Utils/TimeProviderExtensions.cs	100.00% <ø> (ø)
...xtensions/Telemetry/TelemetryResilienceStrategy.cs	100.00% <ø> (ø)
...olly.RateLimiting/RateLimiterResilienceStrategy.cs	100.00% <ø> (ø)
...c/Polly.Core/ResilienceStrategy.Async.ValueTask.cs	100.00% <100.00%> (ø)
.../Polly.Core/ResilienceStrategy.Async.ValueTaskT.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResilienceStrategy.cs	100.00% <100.00%> (ø)
...c/Polly.Core/Utils/ReloadableResilienceStrategy.cs	100.00% <100.00%> (ø)
... and 1 more