ant-tasks.eclass: Use standard copyright/license header.

ant-tasks.eclass: Use standard copyright/license header.

As decided by the council in its 2020-02-09 meeting, ebuilds and
eclasses in the Gentoo repository must be licensed under GPL-2.

Note that stripping out the "or later" clause is possible without
asking authors for their permission:
https://www.gnu.org/licenses/gpl-faq.en.html#AllCompatibility

Restore the original start year 2007, which is also the year when
the eclass was first committed.

Signed-off-by: Ulrich Müller <ulm@gentoo.org>