Enable kAdblockOnlyMode by default

[atuchin-m]

brave/brave-variations#1806

The feature is enabled by Griffin on desktop (100% on Release since 1.88.96):
https://github.com/brave/brave-variations/blob/main/studies/AdblockOnlyModeStudy.json5#L10

Still disabled in the code:
https://github.com/brave/brave-core/blob/v1.93.129/components/brave_shields/core/common/features.cc#L122-L123

Implemented here: brave/brave-core#31357

Proposal:

• Flip kAdblockOnlyMode to FEATURE_ENABLED_BY_DEFAULT so the code default matches Griffin rollout
• Set max_version to Griffin study

[atuchin-m]

@aseren @ShivanKaul PTAL.
Also, prompt_after_shields_disabled_count is different from the code-default.