docs: add deprecation comments for message fields and enums #2602

Job	Run time
docs	40s
mypy	33s
showcase (showcase)	1m 1s
showcase (showcase_alternative_templates)	1m 9s
showcase-unit (3.7)	1m 5s
showcase-unit (3.7, _alternative_templates)	53s
showcase-unit (3.7, _mixins)	55s
showcase-unit (3.7, _alternative_templates_mixins)	57s
showcase-unit (3.8)	1m 8s
showcase-unit (3.8, _alternative_templates)	53s
showcase-unit (3.8, _mixins)	56s
showcase-unit (3.8, _alternative_templates_mixins)	1m 9s
showcase-unit (3.9)	53s
showcase-unit (3.9, _alternative_templates)	1m 0s
showcase-unit (3.9, _mixins)	1m 17s
showcase-unit (3.9, _alternative_templates_mixins)	1m 2s
showcase-unit (3.10)	55s
showcase-unit (3.10, _alternative_templates)	1m 2s
showcase-unit (3.10, _mixins)	57s
showcase-unit (3.10, _alternative_templates_mixins)	1m 3s
showcase-unit (3.11)	49s
showcase-unit (3.11, _alternative_templates)	1m 7s
showcase-unit (3.11, _mixins)	1m 1s
showcase-unit (3.11, _alternative_templates_mixins)	49s
showcase-mtls	0s
showcase-mypy	1m 1s
showcase-mypy (_alternative_templates)	1m 0s
showcase-unit-add-iam-methods	1m 2s
snippetgen	35s
unit (3.7)	59s
unit (3.8)	49s
unit (3.9)	1m 1s
unit (3.10)	56s
unit (3.11)	57s
fragment (3.7)	1m 58s
fragment (3.7, _alternative_templates)	1m 57s
fragment (3.8)	1m 59s
fragment (3.8, _alternative_templates)	1m 57s
fragment (3.9)	1m 58s
fragment (3.9, _alternative_templates)	2m 0s
fragment (3.10)	2m 0s
fragment (3.10, _alternative_templates)	2m 2s
fragment (3.11)	2m 3s
fragment (3.11, _alternative_templates)	2m 1s
integration	1m 45s
goldens-lint	1m 56s
style-check	10s
	55m 20s

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