Moved GTM code to start of <body> in template instead of end of <body>

As requested by Tim Yuen from Panalysis:

2) Move Google Tag Manager code to immediately after the <body> tag
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The Google Tag Manager snippet is implemented in the site, but ideally,
it should be
placed immediately after the opening <body> tag. At the moment, it is
placed before the closing </body> tag.
By having it at the opening <body> tag, the tracking is slightly more
accurate.
Would it be possible to move the snippet to immediately after the
opening <body> tag?