Use Unicode CLDR identifier for Brazilian Portuguese

While packaging this extension for Fedora, rpmlint warned me that pt-br
was an invalid locale directory.  After some investigation, I confirmed
that the Unicode CLDR standard for this is pt_BR, not pt-br.  Other
GNOME software and extensions also use the pt_BR identifier.

https://github.com/unicode-org/cldr/blob/release-43-1/common/main/pt_BR.xml
https://l10n.gnome.org/teams/pt_BR/
https://gitlab.gnome.org/GNOME/gnome-shell/-/blob/45.0/po/pt_BR.po#L33
https://gitlab.gnome.org/GNOME/gnome-boxes/-/blob/45.0/po/pt_BR.po#L25
https://github.com/ubuntu/gnome-shell-extension-appindicator/blob/v57/locale/pt_BR.po#L15

po/pt-br.po → po/pt_BR.po

@@ -7,7 +7,7 @@ msgstr ""
 "PO-Revision-Date: 2021-09-18 16:25-0400\n"
 "Last-Translator: Juarez Rudsatz <[email protected]>\n"
 "Language-Team: \n"
-"Language: pt-br\n"
+"Language: pt_BR\n"
 "MIME-Version: 1.0\n"
 "Content-Type: text/plain; charset=UTF-8\n"
 "Content-Transfer-Encoding: 8bit\n"