Drupal 9 compatibility

Drupal 9 compatibility report: will submit a patch if someone doesn't beat me to it :)

drupal-check -ad web/modules/contrib/google_tag
4/4 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

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Line google_tag.install
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21 Call to deprecated method url() of class Drupal.
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Line google_tag.module
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199 Call to deprecated function drupal_set_message().
232 Call to deprecated function drupal_set_message().
233 Call to deprecated function drupal_set_message().
251 Call to deprecated method strtolower() of class
Drupal\Component\Utility\Unicode.
264 Call to deprecated method strtolower() of class
Drupal\Component\Utility\Unicode.
268 Call to deprecated function drupal_set_message().
298 Call to deprecated function drupal_set_message().
311 Call to deprecated function file_stream_wrapper_valid_scheme().
321 Call to deprecated function drupal_mkdir().
328 Call to deprecated function drupal_chmod().
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Line includes/snippet.inc
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17 Function google_tag_snippets not found while trying to analyse it -
autoloading is probably not configured properly.
19 Function _google_tag_script_snippet not found.
20 Function _google_tag_noscript_snippet not found.
21 Function _google_tag_data_layer_snippet not found.
34 Function _google_tag_script_snippet not found while trying to analyse
it - autoloading is probably not configured properly.
37 Function _google_tag_variable_clean not found.
38 Function _google_tag_variable_clean not found.
39 Function _google_tag_environment_query not found.
76 Function _google_tag_noscript_snippet not found while trying to analyse
it - autoloading is probably not configured properly.
79 Function _google_tag_variable_clean not found.
80 Function _google_tag_environment_query not found.
109 Function _google_tag_data_layer_snippet not found while trying to
analyse it - autoloading is probably not configured properly.
112 Function _google_tag_variable_clean not found.
139 Function _google_tag_environment_query not found while trying to
analyse it - autoloading is probably not configured properly.
146 Function _google_tag_variable_clean not found.
147 Function _google_tag_variable_clean not found.
162 Function _google_tag_variable_clean not found while trying to analyse
it - autoloading is probably not configured properly.
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------ ----------------------------------------------------------------------
Line src/Form/GoogleTagSettingsForm.php
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365 \Drupal calls should be avoided in classes, use dependency injection
instead
380 Function google_tag_snippets not found.
389 \Drupal calls should be avoided in classes, use dependency injection
instead
394 \Drupal calls should be avoided in classes, use dependency injection
instead
429 Call to deprecated function drupal_set_message().
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