Convert Cap(ability) strings to Freeciv21

[jwrober]

Is your feature request related to a problem? Please describe. As part of a purge of legacy freeciv, move the various capability strings to Freeciv21

Describe the solution you'd like Change the following cap strings to Freeciv21

• RULESET_COMPAT_CAP "+Freeciv-3.0-ruleset"
• RULESET_CAPABILITIES "+Freeciv-ruleset-Devel-2017.Jan.02"
• TILESPEC_CAPSTR "+Freeciv-tilespec-Devel-2019-Jul-03
• SPEC_CAPSTR "+Freeciv-spec-Devel-2019-Jul-03"
• SOUNDSPEC_CAPSTR "+Freeciv-3.0-soundset"
• MUSICSPEC_CAPSTR "+Freeciv-2.6-musicset"
• FC_AI_MOD_CAPSTR "+Freeciv-3.1-ai-module-2019.Feb.16"

Describe alternatives you've considered None

Additional context None

[lmoureaux]

Note that this will introduce compatibility problems with Aviation.