webproxy: T6328: Add a warning message about deprecation of web proxy URL filtering

[natali-rs1985]

Change Summary

Add a warning message about deprecation of web proxy URL filtering

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe): Deprecation warning

Related Task(s)

• https://vyos.dev/T6328

Related PR(s)

Component(s) name

service webproxy

Proposed changes

How to test

set service webproxy listen-address 192.168.122.14
set service webproxy url-filtering squidguard block-category 'ads'
commit

vyos@vyos# commit
[ service webproxy ]

WARNING: DB of category ads does not exist.
 Use [update webproxy blacklists] or delete undefined category!


DEPRECATION WARNING: URL filtering with SquidGuard is deprecated and
will be removed in the future VyOS versions.

Smoketest result

vyos@vyos# /usr/libexec/vyos/tests/smoke/cli/test_service_webproxy.py
test_01_basic_proxy (__main__.TestServiceWebProxy.test_01_basic_proxy) ... ok
test_02_advanced_proxy (__main__.TestServiceWebProxy.test_02_advanced_proxy) ... ok
test_03_ldap_proxy_auth (__main__.TestServiceWebProxy.test_03_ldap_proxy_auth) ... ok
test_04_cache_peer (__main__.TestServiceWebProxy.test_04_cache_peer) ... ok
test_05_basic_squidguard (__main__.TestServiceWebProxy.test_05_basic_squidguard) ... ok

----------------------------------------------------------------------
Ran 5 tests in 184.114s

OK

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly