Update dependency com.thoughtworks.xstream:xstream to v1.4.18

[mend-for-github-com]

This PR contains the following updates:

Package	Type	Update	Change
com.thoughtworks.xstream:xstream (source)	compile	patch	1.4.5 -> 1.4.18

By merging this PR, the below issues will be automatically resolved and closed:

Severity	CVSS Score	CVE	GitHub Issue
High	9.9	CVE-2021-21345	#117
High	9.8	CVE-2013-7285	#89
High	9.8	CVE-2021-21344	#2
High	9.8	CVE-2021-21346	#115
High	9.8	CVE-2021-21347	#114
High	9.8	CVE-2021-21350	#98
High	9.1	CVE-2021-21342	#4
High	9.1	CVE-2021-21351	#96
High	8.8	CVE-2020-26217	#51
High	8.8	CVE-2021-29505	#113
High	8.8	CVE-2021-39139	#134
High	8.6	CVE-2021-21349	#111
High	8.5	CVE-2021-39141	#135
High	8.5	CVE-2021-39144	#138
High	8.5	CVE-2021-39145	#137
High	8.5	CVE-2021-39146	#140
High	8.5	CVE-2021-39147	#139
High	8.5	CVE-2021-39148	#142
High	8.5	CVE-2021-39149	#141
High	8.5	CVE-2021-39150	#155
High	8.5	CVE-2021-39151	#131
High	8.5	CVE-2021-39152	#156
High	8.5	CVE-2021-39153	#133
High	8.5	CVE-2021-39154	#132
High	7.7	CVE-2020-26258	#94
High	7.5	CVE-2016-3674	#79
High	7.5	CVE-2017-7957	#100
High	7.5	CVE-2021-21341	#5
High	7.5	CVE-2021-21343	#3
High	7.5	CVE-2021-21348	#112
High	7.5	CVE-2021-43859	#157
Medium	6.8	CVE-2020-26259	#99
Medium	6.3	CVE-2021-39140	#136

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