Cap the max jitter fraction for cache refresh buffer time to 0.5 (#3402)

## Description
Via code inspection, we have identified that there is a potential bug in
`DEFAULT_BUFFER_TIME_JITTER_FRACTION`. Specifically, if the fraction
happens to be set to 1.0, we end up not respecting the buffer time for
cache refresh (see diagrams in #2335). This PR will cap the max fraction
value to 0.5 to avoid the problem.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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Co-authored-by: John DiSanti <jdisanti@amazon.com>