ru-RU: Add @andOlga as maintainer

[andOlga]

Having finished bringing the ru-RU translation up-to-date, I'd like to take on the maintenance of this language in the future so that I can add/change strings as they arrive.

This PR performs all the maintainer modification tasks: changes the GitHub issue templates, the readme, and removes the "unmaintained" comment from data/language/ru-RU.txt.

[github-actions]

Check results

For details go to Translation Check -> Details. Expand Run checks build stage and use the build-in search to find your language (e.g. pl-PL)

	Missing The translation is not added to translation file. (e.g. STR_9999 is in en-GB but is not available in given language)	Not needed The translation file contains entries that are not in en-GB and should be removed (e.g. STR_9999 exists in given language but is not in en-GB)	Same The translation and source string is exactly the same. (e.g. STR_9999 is Umbrella in both en-GB and given language). This may be desired in some cases (e.g. April is the same in English and German)
ru-RU	2	0	110

Other translations

	Missing The translation is not added to translation file. (e.g. STR_9999 is in en-GB but is not available in given language)	Not needed The translation file contains entries that are not in en-GB and should be removed (e.g. STR_9999 exists in given language but is not in en-GB)	Same The translation and source string is exactly the same. (e.g. STR_9999 is Umbrella in both en-GB and given language). This may be desired in some cases (e.g. April is the same in English and German)
ar-EG	187	2	1765
ca-ES	2	1	86
cs-CZ	191	1	140
da-DK	5	1	245
de-DE	7	1	141
en-US	3408	0	1
eo-ZZ	0	1	65
es-ES	40	1	159
fi-FI	7	1	96
fr-CA	3564	0	0
fr-FR	2	1	137
gl-ES	4	3	97
hu-HU	7	1	86
it-IT	5	1	133
ja-JP	7	1	121
ko-KR	2	1	89
nb-NO	96	1	163
nl-NL	0	1	175
pl-PL	0	1	96
pt-BR	0	1	133
sv-SE	61	1	150
tr-TR	425	1	98
uk-UA	91	34	725
vi-VN	287	3	2788
zh-CN	71	1	30
zh-TW	182	1	105