man: Use .rst suffix for sources

It's more common to use the dedicated .rst suffix for reStructuredText
files, so now that the files were ported,
use that.

(Renaming the files separately from changing it is less likely
to confuse git, thus the separate commit)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3454>

man/meson.build

@@ -1,5 +1,5 @@
 custom_target('man page',
-  input: 'gnome-shell.txt',
+  input: 'gnome-shell.rst',
   output: 'gnome-shell.1',
   command: [rst2man, '--syntax-highlight=none', '@INPUT@'],
   capture: true,

subprojects/extensions-tool/man/meson.build

@@ -1,5 +1,5 @@
 custom_target('gnome-extensions.1',
-  input: 'gnome-extensions.txt',
+  input: 'gnome-extensions.rst',
   output: 'gnome-extensions.1',
   command: [rst2man, '--syntax-highlight=none', '@INPUT@'],
   capture: true,