Change the retry backoff type default to constant

[martintmk]

Details on the issue fix or feature implementation

Based on the API review:

The default in RetryStrategyOptions.BackoffType is now Constant.

Type-speciifc options can use different default. For example here:
https://github.com/dotnet/extensions/blob/main/src/Libraries/Microsoft.Extensions.Http.Resilience/Polly/HttpRetryPolicyOptions.cs

We can just do:

public class HttpRetryStrategyOptions : RetryStrategyOptions<HttpResponseMessage>
{
   publc HttpRetryStrategyOptions() => BackoffType = RetryBackoffType.ExponentialWithJitter;
}

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 #1260 (d1be3fc) into main (dcbd172) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1260   +/-   ##
=======================================
  Coverage   83.12%   83.12%           
=======================================
  Files         269      269           
  Lines        6239     6239           
  Branches      976      976           
=======================================
  Hits         5186     5186           
  Misses        844      844           
  Partials      209      209           

Flag	Coverage Δ
linux	83.12% <ø> (ø)
macos	83.12% <ø> (ø)
windows	83.12% <ø> (ø)

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

Impacted Files	Coverage Δ
src/Polly.Core/Retry/RetryConstants.cs	100.00% <ø> (ø)
...c/Polly.Core/Retry/RetryStrategyOptions.TResult.cs	100.00% <ø> (ø)

... and 88 files with indirect coverage changes