Pandas 3.x deprecation warnings: Timestamp.utcnow() and 'd' frequency

[onlsol]

Using yfinance-cache with Pandas 3.x raises deprecation warnings:

1. Timestamp.utcnow() => Timestamp.now('UTC')
2. d frequency string => D

Affected: yfc_ticker.py, yfc_prices_manager.py, yfc_time.py.

The replacement methods are backwards compatible with Pandas 2.x.

[onlsol]

@ValueRaider, PR submitted.

Note: existing users may still see deprecation warnings from cached options.

The file ~/.cache/py-yfinance-cache/options.json (created by yfc_cache_manager.py) stores max_age values in deprecated format. If this file was created before the fix, it contains lowercase 'd' values which trigger warnings.

Quick'n'dirty workaround: delete options.json or manually change 'd' to 'D' in that file