Add Rector and PHP 7.4 rules

[tidal]

As discussed (long ago) we want to use Rector to automatically refactor our code base to be more consistent and automate the upgrade to higher PHP Versions once support for a specific version is dropped.
This PR adds rector as a dependecy and applies rules to upgrade the code base completely to PHP 7.4.
(I will add more rector rules gardually in future PRs, as otherwise this will be a nightmare to review)

• Adds rector as a dev dependency
• Adds rector make targets and inclues them in "make all"
• Applies rector PHP 7.4 rules to the code base
• Adds rector info (and missing deptract info) to DEVELOPMENT.md
• Adds intl extension to dev image for better rector performance

[codecov]

Codecov Report

Merging #739 (17b3792) into main (739f212) will decrease coverage by 0.01%.
The diff coverage is 95.45%.

@@             Coverage Diff              @@
##               main     #739      +/-   ##
============================================
- Coverage     82.55%   82.53%   -0.02%     
  Complexity     1272     1272              
============================================
  Files           146      146              
  Lines          3130     3127       -3     
============================================
- Hits           2584     2581       -3     
  Misses          546      546              

Flag	Coverage Δ
7.4	82.54% <95.45%> (-0.02%)	⬇️
8.0	82.60% <95.45%> (-0.02%)	⬇️
8.1	82.60% <95.45%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/API/Baggage/BaggageBuilder.php	0.00% <0.00%> (ø)
src/Contrib/Jaeger/JaegerTransport.php	86.66% <ø> (ø)
src/SDK/Common/Time/ClockFactory.php	100.00% <ø> (ø)
src/SDK/Common/Time/StopWatchFactory.php	100.00% <ø> (ø)
src/API/Baggage/Propagation/BaggagePropagator.php	100.00% <100.00%> (ø)
src/API/Trace/Propagation/B3MultiPropagator.php	100.00% <100.00%> (ø)
...c/API/Trace/Propagation/TraceContextPropagator.php	100.00% <100.00%> (ø)
src/Context/Context.php	76.66% <100.00%> (ø)
src/Context/Propagation/MultiTextMapPropagator.php	100.00% <100.00%> (ø)
src/Contrib/Jaeger/SpanConverter.php	95.29% <100.00%> (-0.11%)	⬇️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 739f212...17b3792. Read the comment docs.

[tidal]

@bobstrecansky @brettmc
Please trigger the image build job when this is merged to pull in the ìntl extension