feat(senders): add GMT to timestamp message in notifier

[almostinf]

Add GMT to timestamp message in notifier

Also fixed corresponding tests

[codecov-commenter]

Codecov Report

Merging #885 (5ff45cb) into master (96b59d7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 5ff45cb differs from pull request most recent head 6d91b1a. Consider uploading reports for the commit 6d91b1a to get more accurate results

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@@           Coverage Diff           @@
##           master     #885   +/-   ##
=======================================
  Coverage   68.28%   68.29%           
=======================================
  Files         187      187           
  Lines       10449    10452    +3     
=======================================
+ Hits         7135     7138    +3     
  Misses       2905     2905           
  Partials      409      409           

Files Changed	Coverage Δ
datatypes.go	85.15% <100.00%> (+0.19%)	⬆️
senders/discord/send.go	66.66% <100.00%> (ø)
senders/mail/send.go	71.66% <100.00%> (ø)
senders/mattermost/sender.go	90.65% <100.00%> (ø)
senders/msteams/msteams.go	80.45% <100.00%> (ø)
senders/opsgenie/send.go	76.40% <100.00%> (ø)
senders/pagerduty/send.go	72.85% <100.00%> (ø)
senders/pushover/pushover.go	82.19% <100.00%> (ø)
senders/slack/slack.go	61.31% <100.00%> (ø)
senders/telegram/send.go	59.68% <100.00%> (ø)
... and 2 more

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[github-actions]

Build and push Docker images with tag: 2023-08-03.267f7f4

[github-actions]

Build and push Docker images with tag: 2023-08-03.267f7f4