packaging: xml2po is deprecated

[kenhys]

Problem

xml2po is used for generating documentation.
But, it is not packaged in Debian. (removed from bullseye or later)
because of xml2po is not supported python3.

Actual

It is a blocker to update .pot.

po2xml doesn't support multiple xml files as a input, so it does not provide equivalent functionality.

po4a may work, but not tested.

https://www.slideshare.net/nabetaro/po-7177145

Expected

migrate xml2po to alternative one.

[kenhys]

pot generating process will be replaced by:

• generate .am file which includes repeated --master foo.xml
• includes above .am from Makefile.am
• Use po4a-gettextize with `--format xml -p milter-manager.pot --master xxxx

[kenhys]

update-po target will be replaced by:

• Execute po4a-updatepo --format xml --po ja.po --master....

[kenhys]

html-ja-build.stamp target will be replaced by:

po4a-translate --keep --format xml .... --localized