refactor!: remove proxy tiers and Request.forefront

[vdusek]

Removes tiered_proxy_urls from ProxyConfiguration, together with _ProxyTierTracker, ProxyInfo.proxy_tier, Request.last_proxy_tier and the proxy_tier argument of new_url() / new_proxy_info(). JS v4 dropped tiers as well (apify/crawlee#3599).

Also removes Request.forefront, since the tier tracker was the only code that set it. Following apify/crawlee#4157, a retried request and a request replaced by the error_handler go to the back of the queue. A request deferred by a RequestThrottledError is reclaimed at the front, where v1 put it at the back.

The proxy management guide replaces the "Tiered proxies" section with "Switching proxies on retries". That section shows a new_url_function that picks the proxy by request.retry_count. The upgrade guide gets a section for each removal. The example will need an update in #2298, which drops the request argument from new_url_function.

Closes: #2299
Closes: #2300

✍️ Drafted by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (v2@08747f7). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/crawlee/proxy_configuration.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##             v2    #2336   +/-   ##
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  Coverage      ?   94.14%           
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  Files         ?      184           
  Lines         ?    13577           
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  Hits          ?    12782           
  Misses        ?      795           
  Partials      ?        0           

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