Remove CIVICRM_TEMP_FORCE_UTF8; deprecate TempTable::setUtf8()

[mfb]

Overview

This is a followup for #13658 to

• Remove the CIVICRM_TEMP_FORCE_UTF8 constant; and
• Deprecate the TempTable::setUtf8() method as tables should be assumed to use UTF-8 as default character set and collation (but may define columns to use some other character set and/or collation).

After

• Setting CIVICRM_TEMP_FORCE_UTF8 would have no effect. All instances would be assumed to have UTF-8 as the default character set and collation for temporary tables.
• TempTable::setUtf8(FALSE) could still be called, but this would be deprecated / discouraged, because the resulting default character set and collation is undefined (depends on the database default character set and collation). Tables that need to use a specific character set or collation should specify this in the column definition.

[civibot]

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[mattwire]

@eileenmcnaughton I'm happy with this but don't feel comfortable merging as I think it needs opinion from someone on #13658 - thoughts?

[eileenmcnaughton]

@mattwire let's keep it for after 5.14 is forked. In 5.13 we make utf8 the default for all sites already - so this should be effectively 'no-change' - but the next cut is tomorrow & that will give it more rc time

[mattwire]

Merging now that 5.14 RC has been branched.