beta updates with various bugfixes and stability improvements

[Phaze-III]

This PR contains a collection of bugfixes, small improvements and a change in the used Arduino core version against the current beta branch (NRZ-2021-134-B4). Main objective is to get a stable build that might be a candidate for publishing as next beta release. All changes have been extensively tested on three different nodes/sensors under various (network) conditions.

Summary of changes (please see the specific commits for details):

• Go back to Arduino Core 2.7.4 as a workaround for stalling OTA downloads (#915)
• Change setting of compiler optimization to -O3 for better performance
• Replace LetsEncrypt DST Root CA X3 with ISRG Root X1 certificate (#1013)
• Language updates and fixes
• Add displaying errors with config parsing and network connections
• Minor bug fixes and cosmetics (including a fix for #921)
• Update Github Actions CI workflow

Comments and testers are very welcome.

[Phaze-III]

@ricki-z Thanks for merging. Please note however that I haven't had a chance to test the longer term stability of the patches with the SEN5X additions to the beta branch. I do see some instability on one of my sensors now. I'll try to do some debugging the next week.