Replace heuristic printable checker with simple UTF-8/ASCII validator

[NeoCat]

#2236 tried to introduce some code to avoid breaking the Serial console by malformed binary garbage, but the changes were redundant and the garbage was still printed to the console.

• It replaced the heuristic that checked only the first 12 bytes with a full-string check, but this is as costly as a full ASCII / UTF-8 validation.
• It added another heuristic specifically for LOGE(), but to stop logging the buffer content is sufficient.

This PR replaces heuristics with full ASCII/UTF-8 string validation.

• By default, only ASCII printable characters are allowed.
• Defining #define AUDIOTOOLS_METADATA_ICY_ASCII_ONLY false allows valid UTF-8 strings.