Remove unnecessary mutating behavior from Parameters.getCatchall()

[gwynne]

These changes are now available in 4.9.1

Parameters.getCatchall() has historically been mutating, in order to delay the removal of percent encoding from matched catchall values until it is called for the first time. This was an unnecessary optimization which just adds confusion to the behavior of the API and makes it harder to use. We now remove the percent encoding immediately when setCatchall() is called instead.

Additional changes:

• Minimum Swift version updated to 5.8
• set(_:to:) now falls back to the original value if removing percent encoding fails, matching the existing behavior of the catchall logic. This should have no impact in real systems, as an invalid input of this kind should never reach the routing logic.

(N.B.: While it is an API break for a previously non-mutating method to become mutating, the reverse is not the case, so this is a patch-level change.)

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 83.65%. Comparing base (2a92a7e) to head (410ee7b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
- Coverage   83.76%   83.65%   -0.11%     
==========================================
  Files           4        4              
  Lines         234      208      -26     
==========================================
- Hits          196      174      -22     
+ Misses         38       34       -4     

Files	Coverage Δ
Sources/RoutingKit/Parameters.swift	88.23% <87.50%> (+13.23%)	⬆️

... and 1 file with indirect coverage changes