Enable GTFS GraphQL API by default, remove the word "legacy" from its documentation

[leonardehrenfried]

Summary

It's finally happening! This removes the word legacy from the API that is now going to be known as GTFS GraphQL API in the documentation.

It also renames its feature switch to GtfsGraphQlApi and enables it by default.

Issue

Relates to #4828

[codecov]

Codecov Report

Patch coverage: 83.33% and project coverage change: +0.01 🎉

Comparison is base (e6436fa) 65.42% compared to head (df112cd) 65.44%.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #5202      +/-   ##
=============================================
+ Coverage      65.42%   65.44%   +0.01%     
- Complexity     14477    14491      +14     
=============================================
  Files           1755     1754       -1     
  Lines          67987    68022      +35     
  Branches        7231     7238       +7     
=============================================
+ Hits           44482    44515      +33     
- Misses         21035    21042       +7     
+ Partials        2470     2465       -5     

Impacted Files	Coverage Δ
...pentripplanner/api/configuration/APIEndpoints.java	0.00% <0.00%> (ø)
...r/standalone/config/routerconfig/ServerConfig.java	100.00% <ø> (ø)
...ntripplanner/framework/application/OTPFeature.java	87.87% <100.00%> (ø)
...g/opentripplanner/standalone/config/OtpConfig.java	94.11% <100.00%> (ø)
...tripplanner/standalone/config/OtpConfigLoader.java	76.66% <100.00%> (+0.80%)	⬆️

... and 12 files with indirect coverage changes

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